THE EMBRYO 333 



evidence along this line. There has clearly been parallel development 

 of the cotyledon along several or many lines to form an absorbing 

 structure. 



The histological structure of the absorbing parts of the cotyledon 

 among major angiosperm taxa varies greatly; specialization of this part 

 of the cotyledon has followed more than one line. In dicotyledons that 

 have endosperm, the cotyledons, with rare exceptions, remain only 

 temporarily within the seed coats after germination. After the endo- 

 sperm is absorbed, the cotyledons are freed from the seed coats and 

 commonly become more or less leaflike, assimilating organs. Histo- 

 logically, also, they resemble leaves, and their epidermal cells are, in 

 form, like those of most dicotyledons. 



In the monocotyledons, especially in those with hypogeal germina- 

 tion, histological modification of the absorbing tip is much greater than 

 in dicotyledons. Modification consists primarilv of change of form in the 

 epidermal cells; the cells are elongated perpendicular to the surface of 

 the cotyledon and, in some taxa, become papillose and form a distinct 

 absorbing epithelium. Types of epidermal modification characterize 

 major taxa. In the Liliales and Juncaginaceae, the epidermal cells are 

 only slightly or not at all modified. In the grasses, elongation of the 

 epidermal cells is great — up to several or many times their transverse 

 diameter — and the tips of the cells may penetrate, brushlike, into the 

 endosperm. Intermediate types are common in other families. 



Other modifications related to absorption involve shape of the cotyle- 

 don tip itself (Fig. 128). The modified tip (scutellum) may have the 

 general form of the seed itself, as in the grasses (greatly elongated in 

 Zizania, for example), and may retain that form after germination, as 

 in the Commelinaceae. The absorbing tip in many taxa is roughly 

 spherical but, with germination, increases greatly in size, and changes 

 in shape, filling the seed coats, as in the palms, where it may even be- 

 come ruminate, following the convolutions of the endosperm. Club- 

 shaped, cylindrical, and even filiform tips are frequent. Tips of this 

 general form may become completely haustorial (Fig. 128D, H); all 

 cells may be elongated parallel with the long axis, as in the Cyperaceae. 

 In Juncus, the tip is pyriform; its large, little-elongated, epidermal cells 

 resemble the lilialean type. (It is noteworthy that the cotyledon tips 

 of the sedges differ greatly in type from those of the grasses, and that 

 the tip in Juncus is not closely like either of these but resembles that of 

 the lilies. Cotyledon structure fails to support the view that the Junca- 

 ceae are intermediate between lilialean stock and the grasses and sedges, 

 or the view that grasses and sedges are closely related.) 



In the dicotyledons, the cotyledon serves — in addition to storage — for 

 absorption of food and often later for assimilation; in the lower taxa 



