428 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS 



succumb to the embryo, which also contains the diploid number 

 like the nucellus and the integuments. 



Embryo. The most difficult problem in the embryogeny of an- 

 giosperms is the derivation of the monocotyledonous state from 

 the dicotyledonous. In the dicotyledons the two cotyledons are 

 lateral structures and the stem tip is terminal, while in the mono- 

 cotyledons the cotyledon occupies a terminal position and the stem 

 tip is lateral. One view is that the single cotyledon arose by a 

 fusion of two originally separate primordia; the other is that one of 

 the cotyledons became suppressed at an early stage of develop- 

 ment. Each of these views has been supported by a considerable 

 mass of evidence, morphological as well as anatomical. The opin- 

 ion has also been advanced that in certain plants the monocoty- 

 ledonous state has arisen by a division of labor between the two 

 cotyledons, one of which retained the cotyledonary position and 

 function, while the other became modified to form the first plumu- 

 lar leaf. 



It is natural to turn to a study of the development of the embryo 

 as an aid in the solution of this problem. Ontogenetic studies 

 should enable us to find the primordia of the two cotyledons and 

 then to trace the development of one and the arrest of the other, or 

 their fusion into a single member. Not enough work has yet been 

 done to enable a final decision, but a few contributions bearing on 

 this point are briefly referred to here. 



Coulter and Land (1914) found a seedling of Agapanthus umbel- 

 latus (Liliaceae) with two well-developed cotyledons. A study of 

 the embryogeny revealed that, as the proembryo increases in size, 

 its basal or root end remains narrow and pointed while the shoot 

 end widens and becomes broad and flat. Here the peripheral cells 

 begin to divide more actively than the central cells and form a 

 "cotyledonary zone" which assumes a tube-like form with two 

 primordia growing at its tip. Meanwhile the apex of the proem- 

 bryo is left in a depression. Subsequent to this stage, if both the 

 primordia continue to develop equally, two cotyledons are formed. 

 More frequently, however, the cells of one primordium lose their 

 meristematic activity, resulting in a single cotyledon. In other 

 words, both the primordia are present in the beginning, but later 

 the whole growth may be diverted into a single primordium. 

 Looking at the mature stage only, one naturally gets the impression 

 that there is a single terminal cotyledon and a lateral stem tip. 



