THE EMBRYO 327 



of the axis, and of the epiblast. It has, however, commonly been con- 

 sidered of no morphological value, because it is merely a part of the 

 massive late stage of the proembryo, pushed aside by the endogenous 

 root tip as it develops. The presence of similar, smaller sheaths about 

 the bases of adventitious roots in young seedlings supports the view 

 that it is not morphologically significant. A coleorhiza is absent in the 

 simple, primitive monocotyledonous embr^'o. 



The Cotyledons. The cotyledons have been much discussed, since the 

 earliest years of descriptive botany — they were described by Malpighi 

 in the seventeenth century. In the earlier years, interest centered in 

 their morphological nature; in the twentieth century, the relationship of 

 the two of the dicotyledons to the one of the monocotyledons has re- 

 ceived most attention. Cotyledons were early called seedling leaves, 

 storage leaves, organs sui generis, and haustoria "reminiscent of the 

 suctorial 'foot' of pteridophytes." In the form of a scutellum, the cotyle- 

 don has been called merely a lobe or appendage of the radicle, or of 

 the axis. The consistently paired arrangement of the cotyledons — in 

 most taxa, with neitlier one apparently belonging in the phyllotactic 

 spiral of the leaves — has been used to support the sui generis theory. 

 But, in some monocotyledons, the cotyledon seems to fit into the spiral 

 system of appendages. 



Cotyledons are closely leaflike in relation to the axis — in vascular 

 structure, in ontogeny, and in detailed histological structure when 

 they become photosynthetic organs. Leaflike cotyledons that serve only 

 temporarily as storage organs are doubtless the primitive type. Within 

 the angiosperms, there has been added to their basic function of food 

 storage the capacity to absorb the food from the endosperm and, with 

 the endosperm as an intermediary, from the perisperm also. All stages 

 in the development of this function and in the structiual modification 

 of the cotyledon accompanying this change are found. Transformation 

 of the tip of the cotyledon into a temporary absorbing structure, held 

 within the seed coats and later freed from the seed, is a first step toward 

 the transformation of part or all of the cotyledon into a suctorial struc- 

 ture, permanently retained in the seed. Elaboration in form of the 

 cotyledon is extreme in taxa where there is a highly specialized suc- 

 torial "organ" — the scutellum. This modification is an outstanding char- 

 acter of the higher monocotyledons. (The term scutellum is commonly 

 restricted to the embryos of grasses and sedges, but is separable from 

 the retained absorbing tips of cotyledons of other taxa only in degree of 

 specialization. ) 



Apparent dichotomy of the cotyledons — bilobing in some taxa, and a 

 "paired" vascular system in many — led to the theory that cotyledons 

 are primitive organs retained in the embryo, not appendages of leaf 



