Evolution of Plants 369 



reduced to scattered closed strands? Third: How did the 

 trimerous flower of monocotyledons and the dimerous orpen- 

 tamerous flower of dicotyledons arise? 



As regards the first of these questions, it can be said that 

 throughout their entire history, whether amongst recent plants 

 or within the seeds of extinct cordaital plants, the cotyledons 

 clearly functioned as organs of intermediation between the 

 stored food passed in from the parent, and the succeeding 

 embryo plant, during its growth into a seedling. So the two 

 periods of their energizing activities are: first, that period 

 when they are forming on the young embryonic axis up to the 

 stage of seed maturation; second, that period when, under 

 the stimulus of heat, moisture, and oxygen supply, the coty- 

 ledons start to absorb the stored food, to pass it into the grow- 

 ing axis, and often during the process to themselves grow 

 considerably in size. 



Now Henslow's fundamental thesis is, in regard to the origin 

 of monocotyledons from a dicotyledonous ancestry, that trans- 

 fer to an increasingly moist environment brought about the 

 change. In connection there^Adth, and speaking of the prob- 

 able absorption of one of the cotyledons, he says "that the 

 energy of growth is arrested in one and not in the other appears 

 to be the whole interpretation of a monocotyledonous em- 

 bryo." 



But, if lines of growth energy have caused the change, such 

 change should have been effected gradually during the spe- 

 cially active growth-period. Now in nearly all monocotyledons 

 the single seed-leaf is small up to the time of germination, 

 while it increases enormously in size during the germination 

 period. Thus that of a leek or onion seedling is easily one 

 hundred times the size that it was while inside the seed; that 

 of a coconut is several hundred times larger. 



The increased size however is mainly due to great growth 

 in length, to a much less extent and only later, to growth in 

 width of the cotyledon. Now a broad fundamental difference 

 at the present day between dicotyledons and monocotyledons, 

 in root, stem, and leaf, is the steady and diffuse intercalary 



