332 MORPHOLOGY OF THE ANGIOSPERMS 



slightly enlarged, starchless, and retained in the seed but hardly dififers 

 histologically from the starch-bearing lower part of the cotyledon; in 

 Phihjdrum and Pontederia, absence of starch distinguishes a weak, 

 short-lived, suctorial tip. In the Juncaceae, only the tip is suctorial, but 

 it is retained within the seed coats, not withdrawn, as in TriJUiim. The 

 Bromeliaceae also show various stages in this modification in form and 

 function; Tillandsia has no suctorial tip; in Pitcairnia, the foliaceous 

 cotyledon has a short but thick absorbing tip. Puija and Dijckia have a 

 thick cotyledon, which largely fills the seed — there is little endosperm 

 — and remains within the seed coat for weeks, then emerges and be- 

 comes a photosynthetic organ. 



Where the cotyledon tip is retained permanently within the seed 

 coats, the epidermal cells that are in contact with the endosperm are 

 palisadelike. The palisade layer may have a smooth or a papillose sur- 

 face. The absorption of food from the perisperm is also by these 

 specialized epidermal cells, apparently not directly, but through the 

 partially collapsed endosperm cells as intermediaries. 



The absorbing tip of the cotyledon varies greatly in shape. It is 

 commonly cylindrical, club-shaped, or filamentous, but may be laminar 

 or peltate; when very small, it is usually spherical. At germination, the 

 scutellum may enlarge greatly, pushing deeply into the endosperm, as 

 in the palms, sedges, Commelinaceae, Musaceae; its form is controlled 

 by position of the endosperm and shape of the seed. In some primitive 

 monocotyledons, especially the Helobiales, there is no storage endo- 

 sperm and the cotyledon has no absorbing tip. In some highly spe- 

 cialized embryos also — those of orchids and some reduced aquatic taxa 

 — there is no endosperm and no absorbing tip. 



The elaboration of a simple photosynthetic cotyledon, such as that of 

 Paris, to form a high type of suctorial structure, by transformation of 

 the blade into an absorbing structure permanently retained within the 

 seed coats, is only the first and simpler stage in the evolutionary his- 

 tory of the suctorial cotyledon. The division of the cotyledon into parts, 

 with the distal part suctorial, a scutellum, and the proximal part 

 elaborated as a protective structure for the plumule, a coleoptile, ac- 

 companied by adnation to other parts of the embryo, makes the story 

 of change in form, function, and anatomy highly complex. Only com- 

 parison of stages — readily found in difi^erent major taxa, with attention 

 to anatomy — makes the morphology of the highest types clear. 



Critical studies of the mature embryo, of germination, and of seedling 

 structure in the more primitive Liliales should be made to determine 

 the phylogenetic development of the absorbing cotyledon. The Helo- 

 biales, which may also be important in these studies, are perhaps too 

 greatly reduced by their aquatic and semiaquatic adaptations to give 



