252 



THE PLANT WORLD 



ing glory, may be included within this group. The trunk of this tree 

 does not appear to be formed by the successive and progressive activity 

 of a distinct cambium layer as in ordinary hard-v^^ood trees, but is of a 

 herbaceous type, and a new cambium is formed every year external to the 

 bast, which endures only during the season in which it is formed. The 

 entire trunk is soft, generally rich in water and storage material, while 

 the external layers including the bark are succulent and rich in chloro- 

 phyl. The leaves are cast off during the dry season, but flowers are pro- 

 duced in some profusion by the aid of the water and food material stored 

 in the trunk. Acacia greggii, and the mesquite {Prosopis) may be 

 included in this group as well as Parkinsonia. The last-named tree is 

 the "palo verde " of the Mexicans, and the amount of chlorophyl and 

 material constructed by its activity present in the branches is so great 

 that the tree forms a most excellent forage for cattle, horses, and deer, 

 which also eat the branches of Ipomcea arborescens . The leaflets of Park- 

 insonia are often discarded while the petiole remains and carries on the 

 leaf-functions in diminished measure. (Fig. 2.) 





Fig. 2. Plantlet oi Parkinsonia, some of the leaves of which have discarded the leaflets and retained 



the green petioles. 



Another interesting variation in the method of casting of the leaves 

 has been found in the recently discovered Fonquieria viacdougalii. The 

 branches of this tree are very rich in chlorophyl and continue the func- 

 tions of photosynthesis during almost the entire year. The casting of 

 the simple leaves is carried out in such manner however that the petiole 

 is not cut off at its base near the trunk. On the other hand the line of 

 separation begins on the lower (outer side) of the petiole near the basal 



