THE EMBRYO 



335 



of the monocotyledons, it serves for absorption and assimilation; in the 

 higher taxa, for absorption and — as the coleoptile — for the protection of 

 the plumule. The elaboration of the cotyledon in histological structure, 

 as well as in complexity of form — both adaptations primarily to hypogeal 

 germination — supports the view that the monocotyledons are, all in all, 

 more liighly specialized than the dicotyledons. 



The change in function of the tip of the cotyledon and the reten- 

 tion of this part within the seed (Fig. 128) is accompanied by in- 

 creased development of the plumule within the seed; where there is no 



icVA I /B 



Fig. 129. Diagrams of cotyledons of monocotyledons showing evolutionary changes 

 accompanying the change from epigeal to hypogeal germination. A, without a neck; 

 B, C, elongated neck, seed coat retained in C; D to H, successive stages in the 

 downbending and connation of the cotyledonary neck with the sheathing base: 

 G, H, adnation of neck to hypocotyl forming a mesocotyl. x, y, two forms (in face 

 view, split open ) of prolonged shoulders of sheath margins at point of down- 

 bending, shown in G, and H, in side view; x. stipular, and ij, stipular with addition 

 of ligule. 



photosynthetic activity in the cotyledon, early growth of the embryo is 

 dependent upon the first foliage leaves. The cotyledon, not functioning 

 in photosynthesis, tends to retain its tip in the ground; its base, sheath- 

 ing the plumule, elongates as growth continues, the median, connecting 

 part forms a slender uparched connecting "neck" (Figs. 128 and 129). 

 Morphologically, the neck plays an important part in the structural 

 changes involved in the formation of the most specialized embryos. The 

 neck has the form of an inverted U or V, with the absorbing tip at the 

 end of one arm and the sheathing base at the other (Fig. 129E). The 

 distal arm is parallel with the sheathing base and, in evolutionary spe- 

 cialization, comes to be appressed to it and, ultimately, to fuse wltli it 



