6 .MORPHOLOGY OF ANGIOSPERMS 



ent tendency of the spiral groups of Monocotyledons and Dicot- 

 yledons to express the appropriate cyclic number, when the 

 conditions seem to favor indefinite numbers, is even more re- 

 markable than the constant reappearance of the cyclic number 

 in families in which it has become established. Just what has 

 determined these numbers for the two great groups is an inter- 

 esting but unanswered question. The problem is confused by 

 the fact that certain plants, undoubtedly Monocotyledons or 

 Dicotyledons by all the usual tests, have the cycle number of 

 the other group. 



In addition to the distinguishing characters enumerated 

 above, others of much less general application have been sug- 

 gested, but it is not clear that any of them are really significant 

 group characters. 



There are certain general differences in the leaves of the 

 two groups that deserve mention, since they come as near rep- 

 resenting group tendencies as any of the secondary characters 

 just enumerated. Among Dicotyledons the foliage leaf is gen- 

 erally more differentiated than among Monocotyledons, inclu- 

 ding a petiole and often stipules. In fact stipules would be 

 quite characteristic of Dicotyledons were they not lacking in 

 so many, for Monocotyledons possess no such structures. 

 Among the latter, however, there is the almost equally char- 

 acteristic leaf-sheath from which the blade directly arises. 

 This general distinction between the leaves of the two groups 

 must have some unappreciated significance, and suggests that 

 it may represent something as fundamental as do the differ- 

 ences in the embryo and the stele. 



The so-called " germination " of the seed is suggestive of 

 different tendencies in the two groups, but the data seem to 

 be too scanty and indefinite as yet for safe generalization. So 

 far as they do exist, they indicate a tendency in Monocotyledons 

 to free the stem and root tips by the elongation of a portion 

 of the cotyledon, the other portion remaining in contact with 

 the endosperm as a digesting and absorbing organ, very sug- 

 gestive of the " foot " of Pteridophytes ; while in Dicotyledons 

 the tendency is to liberate the growing points and cotyledons 

 by the elongation of the hypocotyl^ and even hypogean cotyle- 

 dons are not related to endosperm as digestive and absorbing 

 organs. 



