362 MORPHOLOGY OF THE ANGIOSPERMS 



leaf. But the suctorial cotyledon and the leaflike one stand at the 

 same level — at the top of the hypocotyl — and their vascular supplies 

 (traces) are equal and opposite. 



In some monocotyledons, there is ontogenetic evidence of the loss of 

 a cotyledon. In Cyi-tanthus, an annular primordium surrounds the 

 terminal stem apex. On this meristematic ring, four projections arise 

 and fuse in pairs, forming two cotyledonary rudiments. One of these 

 ceases to develop; the other enlarges, forming an apparently terminal 

 cotyledon. In Agapanthiis, some embryos have one, others two, cotyle- 

 dons. Both types have an annular cotyledonary primordium around a 

 sunken, terminal stem apex. On this ring, either one or two projections 

 arise forming one or two cotyledons. Where, in a cotyledon, there is 

 ontogenetic change in function and in retention within the seed coat, 

 the entire cotyledon may be transformed simultaneously — Arum, 

 Peperomia — or the change may be progressive from apex to base, ex- 

 tending over some time in the early-seedling stage — Liliales, Helobiales. 

 Relationship of the Monocotyledonous and Dicotyledonous Embryos. 

 The relationship of the two major types— mono- and dicotyledon— and 

 the general morphology of the angiosperm embryo have been much 

 discussed in the twentieth century. But in these discussions, the critical 

 and extensive studies of the 1870s— which dealt especially with the 

 interpretation of the monocotyledonous embryo — have not received the 

 attention they deserve. The earlier interpretations were based on broad 

 comparative studies and provided a base for understanding the highly 

 specialized types— those of the grasses and palms especially— which 

 cannot be soundly interpreted without an understanding of simple 

 forms. With but few exceptions, anatomical evidence has not been used 

 critically to support comparisons of external structure. Today there is 

 litrie agreement as to the nature of some of the major parts of the more 

 specialized monocotyledonous embryo; the use of such terms as scutel- 

 lum, coleoptile, coleorhiza, and epiblast, applied to the most advanced 

 types, obscures the real nature of the parts. 



The major difference in the mono- and dicotyledonous embryos is in 

 the number of cotyledons, a difference that forms an important part of 

 the basis for separating the monocotyledons and dicotyledons. Yet, in 

 this character, as in leaf venation and stelar structure, no clear line of 

 separation can be drawn; among the dicotyledons, several taxa have one 

 cotyledon, and some monocotyledons have vestiges of a second. Most of 

 these aberrant forms have long been known, and described and cited 

 in evidence of the derivation of one type of embryo from the other. 



In the analysis of the evolutionary relationship of the two major types 

 of embryo, two viewpoints have been supported: in one, the dicotyle- 

 donous type is considered primitive, the monocotyledonous derived 

 from it by the fusion of two cotyledons or by the loss (repression) of 



