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(American Journal of Botany. 1999;86:1505-1511.)
© 1999 Botanical Society of America, Inc.

Anatomy of two mechanisms of breaking physical dormancy by experimental treatments in seeds of two North American Rhus species (Anacardiaceae)1

Xiaojie Li 3 2, Jerry M. Baskin 3 and Carol C. Baskin 3,4

3School of Biological Sciences, University of Kentucky, Lexington, Kentucky 40506-0225; and 4Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546-0091

Received for publication October 26, 1998. Accepted for publication April 8, 1999.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Anatomy of the endocarp was studied in relation to the physical dormancy-breaking mechanisms in experimentally treated Rhus aromatica var. aromatica and R. glabra germination units, which include seed plus endocarp (hereafter seeds). The endocarp has three distinct layers, with brachysclereids on the outside, osteosclereids in the middle, and macrosclereids on the inside. Brachysclereids in the carpellary micropyle region (i.e., region immediately adjacent to the integumentary micropyle) are shorter than those in other parts of the endocarp, and the macrosclereids in this region are not elongated. Thus, a weak point is formed in the endocarp. Concentrated sulfuric acid broke seed dormancy in R. aromatica by eroding the brachysclereids and osteosclereids in the carpellary micropyle region, whereas boiling water broke dormancy in seeds of R. glabra by inducing a blister adjacent to the carpellary micropyle.

Key Words: Anacardiaceae • dormancy break • endocarp anatomy • Rhus aromatica • Rhus glabra


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Physical seed dormancy in several angiosperm plant families, e.g., Convolvulaceae, Geraniaceae, Leguminosae, and Malvaceae (Werker, 1997 ; Baskin and Baskin, 1998 ) is caused by a water-impermeable seed coat. However, in a few families, including Anacardiaceae, physical dormancy is caused by a water-impermeable pericarp (Baskin and Baskin, 1998 ). Although the anatomy of impermeability has been studied extensively in many true seeds, especially those of legumes (Barton, 1965 ; Ballard, 1973 ; Rolston, 1978 ; Werker, 1980/1981 ), seeds with an impermeable pericarp have, for the most part, escaped the attention of researchers. For instance, pericarp impermeability was barely mentioned in reviews on physical dormancy by Ballard (1973) , Rolston (1978) , Werker (1980/1981) , Tran and Cavanagh (1984) , or Kelly, van Staden, and Bell (1992) .

Typical of the Anacardiaceae, the endocarp in Rhus aromatica Ait. and in R. glabra L. functions as the seed coat; the true (integumentary) seed coat is not well differentiated and has no mechanical function (Corner, 1976 ). Thus, the germination unit is the seed plus endocarp (hereafter seed). Following maturation desiccation, the endocarp becomes impermeable to water (Li, Baskin, and Baskin, 1999 ), and this is the cause of physical dormancy in these two species. Soaking seeds of R. aromatica for 1 h in concentrated sulfuric acid and those of R. glabra for 1 min or less in boiling water renders the endocarp permeable (Li, Baskin, and Baskin, in press). The purpose of this study was to identify anatomical changes in the endocarp that occur during dormancy break by concentrated sulfuric acid in R. aromatica and by boiling water in R. glabra. Specific objectives were to: (1) describe the anatomical mechanism(s) of certain scarification treatments that break physical dormancy in the seeds; (2) determine the depth of endocarp impermeability; and (3) locate the site of initial water entry to the embryo during imbibition.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
The species
Rhus aromatica Ait. (var. aromatica in this study) and R. glabra L. are dioecious shrubs native to North America. The former species is distributed in the eastern part of the United States and in southern Ontario and southern Quebec in Canada (Barkley, 1937 ), and the latter occurs in all 48 contiguous states and adjacent Canada and Mexico (Little, 1977 ). The fruits are one-seeded drupes ~7.5 x 6.9 mm in R. aromatica and ~5.4 x 4.8 mm in R. glabra; the seeds are 4.7 x 3.9 mm and 3.1 x 2.4 mm, respectively (Li, Baskin, and Baskin, in press).

General anatomy of the endocarp
Prior to endocarp lignification, fruits were collected in 1995 for R. glabra and in 1997 for R. aromatica from Raven Run Nature Sanctuary, Fayette County, Kentucky, USA (for site description, see Li, Baskin, and Baskin, 1999) and preserved immediately in formalin-acetic-alcohol (FAA). The fixed tissue was dehydrated using the tertiary butyl alcohol (TBA) series, infiltrated and embedded in paraffin, sectioned at ~10 ;gmm with a rotary microtome (model 820, American Optical, Buffalo, New York, USA), affixed to glass slides (8 x 3 cm), and stained with a safranin-fastgreen schedule. When staining was complete, a drop of Permount (Fisher Scientific, Fair Lawn, New Jersey, USA) mounting medium was used to affix coverslips to the slides. Slides were placed under a compound microscope (Leica DMRB/E, Leica AG, CH-9435 Heerbrugg, Switzerland), which was connected to a MacIntosh computer (MacIntosh Quadra 650) equipped with the public domain NIH Image program (developed at the U.S. National Institute of Health and available on the Internet at http://rsb.info.nih.gov/nih-image/) for digital image capturing and size measurements.

Due to hardness of the tissue, it is difficult to employ the above procedure for making microslides of developmentally advanced fruits, mature dry fruits in particular. Thus, mature, freshly collected fruits and cleaned seeds were cut with a sharp razor blade and placed under a dissecting microscope (model Leica Wild MZ8, Leica AG, CH-9435 Heerbrugg, Switzerland), which was connected to the computer described above, for digital image capturing and size measurements.

Effects of scarification treatments on endocarp structure
Anatomical changes in the endocarp of seeds of R. aromatica and of R. glabra boiled in water for 1 min or soaked in concentrated H2SO4 for 1 h at ambient temperature (~23°C) were studied under the same dissecting microscope.

Layer(s) of endocarp responsible for impermeability
The endocarp of a mature, cleaned seed contains three layers of cells. From outside to inside, they are brachysclereids, osteosclereids, and macrosclereids. Removal of brachysclereids does not allow the seeds to imbibe water (see Results). Thus, an attempt was made to determine whether removal of both brachysclereids and osteosclereids would lead to imbibition. Using a sharp razor blade, brachysclereids and osteosclereids, or these two layers plus part of the macrosclereids, were removed from the seed. These seeds then were placed in petri dishes on moist sand under ambient temperature (~23°C) and light conditions for a period of 1 wk and 1 d for R. aromatica and R. glabra, respectively. At the end of the imbibition period, the seeds were classified as either imbibed or non-imbibed. An imbibed seed easily can be distinguished from a non-imbibed one; the former is considerably larger than the latter.

Site of water entry during imbibition
Microscopical observations revealed that in R. aromatica and in R. glabra the area corresponding to the placenta (hereafter scar, see Results) and the site where the radicle protrudes during germination (hereafter carpellary micropyle, see Results) differ from the rest of the endocarp. Thus, a closer look was taken at these two areas in relation to water uptake during imbibition.

A total of 500 seeds of R. aromatica that had been scarified effectively by soaking in concentrated H2SO4 for 1 h were divided into five groups of 100 seeds each for treatments as follows: (1) no blocking applied (control); (2) blocking material applied to scar area (i.e., scar plus surrounding exudates); (3) blocking material applied to carpellary micropyle area; (4) blocking applied to entire seed except carpellary micropyle area; and (5) blocking applied to entire seed.

Water-repellent nail slick (Noxell Corp., Hunt Valley, Maryland, USA) was used as the blocking material. The seeds were left to surface-dry for 2 d and then placed in petri dishes on moist sand at room temperature (~23°C) for 7 d, at which time they were classified as either fully imbibed or not imbibed.

A similar experiment was conducted with R. glabra seeds that first were boiled in water for 1 min. Since a blister is formed adjacent to the carpellary micropyle in seeds of this species (see Results), some of the treatments differed from those applied to R. aromatica. Treatments were as follows: (1) no blocking applied (control); (2) blocking material applied to scar area; (3) blocking material applied to carpellary micropyle plus blister area; (4) blocking material applied to entire seed except carpellary micropyle plus blister area; (5) blocking material applied to entire seed; (6) blocking material applied to carpellary micropyle area only; and (7) blocking material applied to blister area only. Additionally, seeds were imbibed for 1 d instead of 7 d.

Amount of water taken up at room temperature (~23°C) by R. glabra seeds treated as described above was determined, using 80 seeds per blocking treatment. The seeds were weighed individually to the nearest 0.0001 g before being placed into the petri dishes on moist sand. Twenty-four hours after the beginning of the imbibition experiment, each seed was reweighed and percentage of water uptake calculated.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
General anatomy of the endocarp
Two areas on the endocarp differ anatomically from the remainder of the endocarp. One area corresponds to the origin of the funiculus, by which the ovule is attached to the ovary. The funiculus appears to originate from the mesocarp, and it penetrates the endocarp (Fig. 1a). Hereafter, this area is referred to as the scar. On the mature cleaned seed (i.e., after it has been detached from the mesocarp of the fruit), the scar is covered by whitish exudates, which appear as an elongated white line (Fig. 1b).



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  		Fig. 1. Special features of the endocarp of Rhus glabra (a–f), showing funiculus (FU) arising from the mesocarp (MC) and penetrating through the endocarp (EC) (a); scar (SA) (see text for explanation) surrounded by whitish exudates running longitudinally (b); carpellary micropyle (CM) (see text for explanation) opening (c), through which the radicle (RC) (d) protrudes during germination; radicle at torpedo stage with tip pointing toward the integumentary micropyle (IM) (e); cross section of CM region with elongated brachysclereids (BS) and non-elongated macrosclereids (MS) 3 wk after anthesis (f) and of a mature, cleaned seed of R. aromatica (g)

 
The second unique area on the endocarp is the site of rupture during germination (Fig. 1c). It is this site from which the radicle (Fig. 1d) protrudes during germination (Li, unpublished data). We refer to this area as the carpellary micropyle, since it is immediately adjacent to the radicle, whose tip points toward the integumentary micropyle (Fig. 1e). The closer the macrosclereids are to the carpellary micropyle, the shorter they are (Fig. 1f). At the carpellary micropyle, the macrosclereids are not elongated at all, resulting in a slit-like feature in the mature seed (Fig. 1g). Further, the brachysclereids at the carpellary micropyle are elongated less than they are in other parts of the endocarp (Fig. 1f, g). When a mature seed is cut longitudinally into two halves, the carpellary micropyle portion of the endocarp appears to be much narrower in thickness than the rest of the endocarp (Fig. 1d). This portion runs 0.62 ± 0.02 (mean ± SE, N = 50) mm in R. aromatica and 0.36 ± 0.01 in R. glabra, which is equivalent to ~5.3 and 4.4% of the circumference of their endocarps, respectively.

Effects of scarification treatments on endocarp structure
Boiling seeds of R. glabra in water immediately resulted in a white-brownish elongated blister ~590 µm long and 129 µm wide (Fig. 2a) on the endocarp adjacent to the carpellary micropyle (Fig. 2b, c). The blister was composed of brachysclereids and osteosclereids (Fig. 2d) that were severed from the macrosclereids and thus uplifted (Fig. 2c, d), leaving the macrosclereids intact (Fig. 2d). The blister was torn apart longitudinally, resulting in a slit (Fig. 2c). No such structure was present on non-boiled seeds (Fig. 2e), nor did one appear on boiled R. aromatica seeds (Fig. 2g). Rather, an opening was formed in the carpellary micropyle area in ~23% of the seeds of R. aromatica that responded to boiling in water (Fig. 2f).



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  		Fig. 2. Effects of boiling for 1 min in water on the endocarp of Rhus glabra (a–e), showing the induced blister (BL) bordering the carpellary micropyle (CM) (a, b, c), uplifted and torn apart from the rest of the endocarp, which resulted in a slit (SL) (c), and exposure of the intact macrosclereids below (d) in a boiled seed, in comparison to the absence of the blister on a non-boiled seed (e); and on the endocarp of R. aromatica seeds that did (f) (note opening at carpellary micropyle area) and did not (g) respond to the treatment

 
Both brachysclereids and osteosclereids at the carpellary micropyle of R. aromatica seeds soaked in sulfuric acid were eroded away (Fig. 3a), thus exposing the neighboring macrosclereids (Fig. 3b), which were darkened, apparently by acid damage. This was quite a contrast to the control R. aromatica seeds, in which the brachysclereids and osteosclereids were present at the carpellary micropyle, thus preventing the seed from absorbing water (Fig. 3e). In areas other than the carpellary micropyle (Fig. 3c), including the scar area (Fig. 3d), only the brachysclereids were eroded away, leaving the osteosclereids and macrosclereids intact. However, in R. glabra seeds that remained impermeable to water after the sulfuric acid treatment only the brachysclereids at the carpellary micropyle were eroded; the osteosclereids were undamaged (Fig. 3f). In some areas, even the brachysclereids remained intact (Fig. 3f) after seeds were soaked in concentrated H2SO4.



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  		Fig. 3. Effects of soaking seeds of Rhus aromatica (a–d) in concentrated H2SO4 for 1 h on endocarp anatomy, showing the darkened macrosclereids (MS) in the carpellary micropyle region (CM), due to the erosion of brachyslcereids and osteosclereids (a, b); osteosclereids (OS) and MS remain intact in other regions (c), including the scar area (see text for explanation) (d), in comparison to the intact endocarp layers in a control R. aromatica seed (e); and erosion of only the brachysclereids even at the CM in R. glabra (f)

 
Layer(s) of endocarp responsible for impermeability
In R. aromatica seeds, removal of the two outer layers only (i.e., brachysclereids and osteosclereids) did not render the seeds permeable to water (Fig. 4a). However, water penetrated through the endocarp if part of the macrosclereid layer also was removed (Fig. 4c). The same was observed in R. glabra seeds (Fig. 4b, d).



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  		Fig. 4. Depth of impermeability in the endocarp, showing a portion of a non-imbibed seed of Rhus aromatica (a) and of R. glabra (b) after removal of only the brachysclereids (BS) and osteosclereids (OS), in contrast to imbibed seeds of R. aromatica (c) and R. glabra (d) due to the removal of both BS and OS plus part of the macrosclereids (MS). Blocking either the carpellary micropyle (CM) (e) or blister (BL) (f) prevented boiled R. glabra seeds from imbibing water

 
Site of water entry during imbibition
In R. aromatica, blocking either the carpellary micropyle of the scarified seed or the entire scarified seed with water-repellent material resulted in <10% of the seeds imbibing water. In contrast, almost 100% of the scarified seeds became fully imbibed even when the scar area or all of the seed except the carpellary micropyle was blocked, as was the case for scarified seeds when no blockage was applied (Fig. 5a). In R. glabra, <5% of the seeds imbibed water when blockage was applied to either the carpellary micropyle, blister, or both, whereas 100% of the seeds became fully imbibed in 1 d when no blockage was applied or when it was applied to areas other than the carpellary micropyle plus blister (Fig. 5b). In accordance with this, the amount of water taken up by scarified R. glabra seeds after 1 d was <2% of their initial mass when either the carpellary micropyle, blister, or entire seed was blocked. On the other hand, water uptake in seeds with no blocking, or in those blocked in areas other than the carpellary micropyle or blister, was >50% (Fig. 5c).



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  		Fig. 5. Effects of blockage with water-repellent nail slick in various areas of the endocarp on imbibition of Rhus aromatica seeds soaked in concentrated H2SO4 for 1 h (a) and of R. glabra seeds boiled in water for 1 min (b), and water uptake of R. glabra seeds boiled in water for 1 min (c). A, control (no blockage); B, scar area blocked; C, carpellary micropyle (a) or carpellary micropyle plus blister (b, c) blocked; D, all of seed except carpellary micropyle blocked (a) or carpellary micropyle plus blister (b, c) blocked; E, entire seed blocked; F, only carpellary micropyle blocked; and G, only blister blocked

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Blockage of water uptake by a water-repellent substance in the carpellary micropyle area reinstated impermeability in H2SO4-scarified seeds of R. aromatica, while blockage in either the scar area or of the entire seed, except the carpellary micropyle area, resulted in almost all seeds imbibing water. This suggests that the carpellary micropyle serves as the site of water entry into these seeds. It also indicates that scarification treatments broke physical dormancy by acting on the carpellary micropyle, which anatomically is the weak point of the endocarp. At the carpellary micropyle, not only are the macrosclereids not elongated, but the brachysclereids also are shorter than they are in other parts of the endocarp (Fig. 1f, g). Consequently, H2SO4 eroded both brachysclereids and osteosclereids at the carpellary micropyle of R. aromatica (Fig. 3a, b), whereas only the brachysclereids were removed elsewhere on the endocarp (Fig. 3c). As a result, water passes through the endocarp of R. aromatica seeds via the carpellary micropyle (Fig. 6b). Although R. glabra seeds have the same anatomy as those of R. aromatica (Fig. 6a), the osteosclereids at the carpellary micropyle in R. glabra are not removed easily by concentrated H2SO4 (Fig. 3f), implying that the chemistry of these cells in this species likely is different from that in R. aromatica. Thus, in R. glabra, except for removal of the brachysclereids, there are no distinguishable differences in a cross section of the endocarp of a control seed and of one that has been soaked in concentrated H2SO4, as also has been reported for R. typhina (Bogacinski and Molski, 1969 ).



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  		Fig. 6. Schematic representation of a longitudinal section of a Rhus seed (a), showing the mechanism of breaking physical dormancy by soaking R. aromatica seeds in concentrated H2SO4 for 1 h (b) and by soaking R. glabra seeds in boiling water for 1 min (c). BL, blister (uplifted brachysclereids and osteosclereids adjacent to the carpellary micropyle); BS + OS, brachysclereids plus osteosclereids; CM, carpellary micropyle; CT, cotyledons; FU, funiculus penetrating the endocarp; MS, macrosclereids; RC, radicle; SC, seed coat; SL, slit due to tearing of uplifted BS + OS, through which water enters the seed; WE, whitish exudates covering the scar

 
According to Stone and Juhren (1951) , water penetrates both the brachysclereids and macrosclereids, and only the osteosclerieds in R. ovata are impermeable to water. In contrast, removal of both the brachysclereids and osteosclereids in R. aromatica and in R. glabra seeds did not make them permeable to water (Fig. 4a, b). Water uptake occurred only when part of the macrosclereid layer also was removed (Fig. 4c, d). The fact that covering the carpellary micropyle area with water-impermeable nail slick prevented boiled R. glabra seeds from imbibing water (Fig. 4f) is also an indication that water cannot penetrate through the macrosclereids.

Boiling breaks physical dormancy in seeds of R. glabra by inducing a blister adjacent to the carpellary micropyle (Fig. 2a, d), without physically changing any other area on the endocarp. Covering either the carpellary micropyle (Fig. 4e) or blister (Fig. 4f) with nail slick reinstates impermeability to water in the boiling water-scarified seeds. These results suggest that both carpellary micropyle and blister are involved in initial water uptake. Since boiling does not alter the anatomy at the carpellary micropyle, it follows that blister formation is the first step in initiating water uptake during imbibition. Thus, we propose that water enters the seed initially through the slit, which resulted from tearing of the uplifted blister, but does not penetrate further due to the intact, water-impermeable macrosclereids below it. Water then flows on the surface of the macrosclereids down the length of the blister. When water reaches the carpellary micropyle, it moves into the embryo (Fig. 6c), since there are no macrosclereids in this area and the seed coat is permeable to water; thus, the seed eventually becomes fully imbibed. Interestingly, when soaked in water under ambient laboratory conditions for a long period of time, the endocarp in a high percentage of R. aromatica seeds, but not in those of R. glabra, gradually becomes permeable (Li, Baskin, and Baskin, in press), reinforcing our speculation that the chemistry of brachysclereids and osteosclereids is different in these two species.

Boiling in water breaks physical dormancy of seeds of some Acacia spp. by causing many randomly located cracks in the seed coat, which then act as sites of water entry (Brown and Booysen, 1969 ). In contrast to such randomness, in R. glabra boiling works specifically on an area near, but not on, the carpellary micropyle, the weak point of the endocarp. If the brachysclereids and osteosclereids at the carpellary micropyle were uplifted in response to boiling water, as are those in the blister area, high temperatures could damage the delicate radicle, which is immediately below the carpellary micropyle.

Functionally, the boiling-induced blister in R. glabra seeds is similar to the strophiolar plug in seeds of the Leguminosae—Albizia lophantha (Dell, 1980 ), Trifolium subterraneaum (Hagon and Ballard, 1970 ), Acacia kempeana (Hanna, 1984 ), and Leucaena leucocephala (Serrato-Valenti, De Vries, and Cornara, 1995 ); the chalazal plug in seeds of the Malvaceae—Gossypium hirsutum (Christiansen and Moore, 1959 ), Abutilon theophrasti (LaCroix and Staniforth, 1964 ), and Sida spinosa (Egley and Paul 1981, 1982 ); and the micropylar plug in seeds of the Convolvulaceae—Convolvulus spp. (Koller and Cohen, 1959 ). In all cases, a predefined part of the embryo envelop responds to high temperatures by lifting away from the rest of the structure, thus acting as the passage for initial water uptake. In Sida spinosa, cells in this raised part of the seed coat are less lignified, contain more hemicelluloses, and have a larger cell lumen than those in other areas of the seed coat (Egley, Paul, and Lax, 1986 ). At this point, we do not know why cells in the blister area in R. glabra respond differently to boiling in water than do those in other parts of the endocarp. Nor do we understand why no blister is induced in R. aromatica seeds treated similarly, or why an opening occurred at the carpellary micropyle area instead. As in Acacia kempeana seeds (Hanna, 1984 ), covering this uplifted structure with water-repellent materials prevents the seed from imbibing water. It is interesting that macrosclereids in boiled R. glabra seeds remain intact, whereas their counterparts in anatomically true seeds are altered physically; for example, macrosclereids separate from one another in Coronilla varia (Brant, McKee, and Cleveland, 1971 ) and in Melilotus alba (Hamly, 1932 ).

In summary, in R. aromatica and in R. glabra anatomy at the carpellary micropyle region differs from that of the rest of the endocarp by having shorter brachysclereids and non-elongated macrosclereids, and this region is the weak point of the endocarp in the mature seed. Water enters at the carpellary micropyle region during imbibition, and this is where the radicle protrudes during germination. This pattern of opening in a limited region of the endocarp also is reported in R. lancea (von Teichman and Robbertse, 1986 ) and is simpler than that of other members of the Anacardiaceae (Hill, 1933, 1937 ). Boiling in water breaks physical dormancy of R. glabra seeds by immediately causing a blister to form adjacent to the carpellary micropyle, but this does not occur in R. aromatica seeds. On the other hand, sulfuric acid renders R. aromatica seeds permeable by eroding the brachysclereids and osteosclereids at the carpellary micropyle in R. aromatica seeds, but not in those of R. glabra. Thus, the chemical make-up of the brachysclereids and osteosclereids near and at the carpellary micropyle must be different between these two species with very similar endocarp anatomy. Rhus trilobata and R. typhina are similar to R. aromatica and R. glabra, respectively, with regards to their response to soaking in sulfuric acid and in boiling water (Li, Baskin, and Baskin, in press).


   FOOTNOTES
 
1 The authors thank Drs. R. L. Geneve and D. N. McLetchie, University of Kentucky, for use of microscope facilities. Back

2 Author for correspondence. Back


   LITERATURE CITED
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Ballard, L. A. T. 1973 Physical barriers to germination. Seed Science and Technology 1: 285–303.

Barkley, F. A. 1937 A monographic study of Rhus and its immediate allies in North and Central America, including the west Indies. Annals of the Missouri Botanical Garden 24: 263–460.[CrossRef]

Barton, L. V. 1965 Dormancy in seeds imposed by the seed coat. In W. Ruhland [ed.], Encyclopedia of Plant Physiology XV/2, 727–745. Springer-Verlag, Berlin.

Baskin, C. C., and J. M. Baskin. 1998 Seeds: ecology, biogeography, and evolution of dormancy and germination. Academic Press, San Diego, CA.

Bogacinski, B., and B. Molski. 1969 Morphology and anatomy of the sumac fruit—Rhus typhina L., Tourn. Rocznik Dendrologiczny 23: 165–183 (Polish with English summary).

Brant, R. E., G. W. Mckee, and R. W. Cleveland. 1971 Effect of chemical and physical treatment on hard seed of penngift crown-vetch. Crop Science 11: 1–6.

Brown, N. A. C., and P. De V. Booysen. 1969 Seed coat impermeability in several Acacia species. Agroplantae 1: 51–60.

Christiansen, M. N., and R. P. Moore. 1959 Seed coat structural differences that influence water uptake and seed quality in hard seed cotton. Agronomy Journal 51: 582–584.[Abstract/Free Full Text]

Corner, E. J. H. 1976 The seeds of dicotyledons, vols. 1, 2. Cambridge University Press, Cambridge.

Dell, B. 1980 Structure and function of the strophiolar plug in seeds of Albizia lophantha. American Journal of Botany 67: 556–563.[CrossRef][ISI]

Egley, G. H., and R. N. Paul. 1981 Morphological observations on the early imbibition of water by Sida spinosa (Malvaceae) seeds. American Journal of Botany 68: 1056–1065.[CrossRef][ISI]

———, and ———. 1982 Development, structure and function of subpalisade cells in water impermeable Sida spinosa seeds. American Journal of Botany 69: 1402–1409.[CrossRef][ISI]

———, ———, and A. R. Lax. 1986 Seed coat imposed dormancy: histochemistry of the region controlling onset of water entry into Sida spinosa seeds. Physiologia Plantarum 67: 320–327.[CrossRef]

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