DEVELOPMENT OF PLANTS 429 



In endospermous seeds it should be noted that the cotyledons 

 are not developed as digestive and absorbing organs as observed 

 in the monocotyledons (page 401). The primary root often per- 

 sists, forming the main or tap root of the plant, which manner of 

 growth is not so common in the monocotyledons. The Dicoty- 

 ledons 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 61,000 

 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, as though 

 demanding for their existence only light and water. The poplars 

 are less restricted and some can endure moderately arid condi- 

 tions. Few plants have greater vitality. A bit of a twig and 

 often a portion of a root is capable of developing buds and so 

 starting the shoot. Trees are often pointed out that have origi- 

 nated through the careless sticking of a twig in the soil and 

 several of the willows are naturally propagated by their twigs 

 which are easily broken off by the winds. Their unusual ability 

 to form numerous buds is well shown in the pollarded willows 

 where the branches have been cut back and a large number of 

 shoots develop about the wound. Perhaps the stimulus of the 

 wound also awakens some of the dormant buds (page 75). The 

 flowers, as in Typha, are arranged on an elongated axis forming 

 a compact inflorescence known as an ament or catkin (Fig. 297, 



