260 MORPHOLOGY OF THE ANGIOSPERMS 



ber of integuments is not a valuable character in the determination of 

 natural relationships, as shown by differences in many undoubtedly 

 related groups: most species of Popiilus have two integuments, but two 

 species and the related genus Salix have only one; in the Piperaceae, 

 Peperomia has one integument and the other genera, two. 



Ovules with two integuments characterize the more primitive dicotyle- 

 dons and the monocotyledons; the gamopetalous dicotyledons have only 

 one. But there are marked exceptions to this distribution. In the Ranun- 

 culaceae and Rosaceae, there are genera with one integument; in the 

 gamopetalous dicotyledons, the Primulaceae and Cucurbitaceae have 

 two integuments. Closely related genera in the Ranunculaceae, Rosaceae, 

 and Ericaceae have different numbers of integuments. Achene types 

 usually have one integument. The single integument seems clearly to 

 represent the advanced condition. The presence of one massive in- 

 tegument in the Sympetalae has been considered support for a mono- 

 phyletic origin for this taxon. 



The outer integument has normal stomata similar to those on the 

 ventral carpel wall and, rarely, may have chlorophyllaceous tissue. 



In some taxa, epidermal cells of the inner integument become spe- 

 cialized and form a nutritive layer, the endothelium or integumentary 

 tapetum, around the archesporium and, later, around the embryo sac, 

 taking the place of disorganizing nucellar tissue. During seed develop- 

 ment, the inner integument is usually destroyed; the "seed coat" is 

 formed by the outer integument alone. The endothelium is well devel- 

 oped in the Rosaceae and Hamamelidaceae and has been called a tape- 

 tum, the equivalent of the microsporangiate tapetum. Unfortunately, 

 the term endothelium has been applied loosely to tissues of both the 

 nucellus and the integument — to the uniseriate sheathing nucellus, which 

 disorganizes as the mother cell divides and the megaspore germinates, 

 and to the histologically modified and prominent inner epidermis of 

 the integument which surrounds the embryo sac and developing embryo. 

 The integumentary layer may become thick-walled and a part of the 

 protective layers of the seed. 



The ovule was early described as crassinucellate, where the nucellus 

 is massive (Fig. 99) and the megaspore mother cells are deep in the 

 distal tissue; and tenuinucellate, where small and delicate and the spore 

 mother cells (usually one) are directly below the epidermis (Fig. 100). 

 All gradations between these types are found. In tenuinucellate types, 

 even in the same species, the spore mother cell may lie directly below the 

 epidermis or be separated from it by one or two cells which belong to 

 the subepidermal tissue or have been formed by periclinal divisions 

 of the epidermis itself. The more massive nucelli, especially those of 

 the Amentiferae, often show, at the base and center, an area of spe- 



