cook: seedling morphology 421 



phologically equivalent units of structure, the phytomers, as 

 they have been called. A typical, vegetative phytomer is 

 represented by one of the internodes, or sections of a jointed 

 plant stem, together with the roots that may grow from the 

 surface of the internode and the leaf that it bears at the distal 

 end. A terminal bud, and usually one or more lateral buds, are 

 formed within the sheathing base of the leaf, so that a succession 

 of phytomers can be produced. 2 



Cotyledons are leaves of the first phytomers of young plants, 

 the internode element of this phytomer being represented by the 

 so-called hypocotyl, or basal joint of the plant stem, from which 

 the roots grow down. Other joints of the stem send out roots 

 from lateral surfaces. In some palms the roots are confined to 

 the ends of the internodes but in others they grow adventi- 

 tiously from any part of the surface. 



GERMINATION IN PALMS AND GRASSES 



Germination commonly begins with the elongation or down- 

 ward growth of the hypocotyl and the roots. In palms and 

 grasses, the development of the hypocotyl is relatively slight, a 

 biological deficiency that has been made good in different ways 

 in the two groups. The grasses have small seeds which are 

 easily buried, and germinate quickly in a few days, whereas the 

 palms have the largest of all seeds and germination is a process 

 of weeks or months. 



The method of germination of some of the large-seeded palms 

 is most remarkable. The function of the hypocotyl in dicotyle- 

 donous plants is performed by a petiole or elongated base of the 

 cotyledon, which grows out of the seed and burrows into the 

 soil, taking the plumule with it. In some cases the cotyledon 

 elongates to the extent of several inches. In the germination 

 of the so-called double-coconut Lodoicea seychellarum, the lar- 

 gest of all seeds, the cotyledon attains a length of several feet. 

 After the burrowing cotyledon of a palm has grown to its full 

 length, the tip, representing the hypocotyl, sends out roots, 



2 Cook, O. F. Morphology and evolution of leaves. Journ. Washington Acad. 

 Sci.. 6: 537. 1916. 



