39 6 THE SALICALES 



embryo which usually consists of a root, stem, and two laterally 

 attached cotyledons. The region of the stem above the cotyle- 

 dons is known as the epicotyl and frequently appears as a minute 

 bud, the so-called plumule. The region of the stem below the 

 cotyledons, the hypocotyl, terminates in the root (Fig. 296). 

 The elongation of the basal portion of the hypocotyl frees the 

 root from the seed and the growth of the upper region of the 

 hypocotyl pushes up into the air the cotyledons and growing point 

 or epicotyl. The formation of the stem is sometimes due to 

 the elongation of the epicotyl alone, the cotyledons frequently 

 containing the storage foods for the nourishment of the young 

 plant and remaining buried in the seed. 



In endospermous seeds it should be noted that the cotyledons 

 are not developed as digestive and absorbing organs as observed 

 in the monocotyledons (page 122). Unlike the latter group, the 

 primary root often persists, forming the main or tap root of the 

 plant. The Dicotyledons include two rather distinct series: a. 

 The Choripetalae, distinguished by their free petals or lack of 

 perianth, b. The Sympetalae, with united petals. 



Series a. Choripetalae 



137. General Characters. This group comprises about 60,000 

 species and includes the majority of the trees and shrubs found 

 in the temperate regions. The flowers as a rule are of a simple 

 type but exceedingly variable in structure, so that it is not pos- 

 sible to separate them into so sharply characterized orders as in 

 case of the monocotyledons, or in the following group of Sym- 

 petalae. Furthermore, these orders doubtless represent many 

 parallel lines of development that are imperfectly understood. 

 For this reason, the order of the presentation in the following 

 pages does not attempt to represent the real relationship of the 

 groups. 



138. Salicales, the Willow and Poplar Order. The willows 

 are almost universally distributed along water ways, apparently 

 demanding for their existence only light and water. The poplars 

 are less restricted and some can endure moderately arid condi- 



