THE PLANT WORLD 255 



climbing stems die back as the rains cease and the plant enters into 

 another resting period. When in this inert condition it may be thrown 

 about like so many pieces of wood without injury. Tubers of guarequi, 

 which were obtained from Torres, Sonora, in February, 1902, have lain 

 about on the metal benches in the greenhouses in the New York Botanical 

 Garden, and occasionally start into activity. A number placed in the 

 exhibition cases in the Museum sent out vines in 1902, and in August, 

 1903, eighteen months after being lifted from the soil, a second 

 crop of tendril-bearing stems were produced in the museum cases. (See 

 Plate XVII, Publication 6, Carnegie Institution, Washington, 1903.) 



A large number of examples of this general type is offered by the 

 flattened and cylindrical opuntias and cereuses, in which the leaves are 

 reduced and quickly fall off, while the much reduced shoot is made of 

 stems and branches containing a comparatively large proportion of suc- 

 culent tissue. The amount of water which may be stored in such tissues 

 may be sufficient for all of the needs of the plant for many months and 

 perhaps more than one season. It is characteristic of this type that the 

 water-storage tissue is found throughout the entire shoot, so that when 

 a single section or ' ' joint ' ' of such a stem is detached it carries with it 

 a supply that may enable it to propagate the plant under the most arid 

 conditions. The presence of spines on nearly all of the plants of this 

 type prevents their extermination by animals, which would otherwise 

 soon destroy them for the water to be obtained from them. During 

 seasons of extreme drought, ranchers sometimes cut great numbers of 

 opuntias and burn off the spines by means of fires of brushwood and then 

 feed the denuded branches to cattle and other animals. (PI. 36.) 



The large barrel-shaped echinocactuses consist of a great swollen 

 stem and a root-system penetrating the rocky and sandy soil in all direc- 

 tions. It is impossible to calculate the practical storage capacity of these 

 plants, but even the most casual inspection would tend to show that a 

 supply of water equal to the entire transpiration of the plant for years is 

 kept in reserve. The Indians of southern California, Arizona, and 

 Mexico habitually make use of this supply when traveling in arid regions 

 away from pools and streams. (PI. 34.) 



The yuccas may be used to illustrate a different type of plant exhibi- 

 ting water storage. In many of the species of this family huge rosettes, 

 or crowns of long, tapering, fleshy leaves are formed, which in some 

 species have a capacity for the storage of large quantities of water and 

 food-material. The arrangement of the sharp-pointed leaves, and the 

 further devices of cutting edges and spines prevent plants of this type 

 also from being plundered by animals for the sake of the water. (PI. 33.) 

 Water storage in leaves is also exhibited by Lycium frevioyitii and other 

 species of the genus, the small obovoid leaves of these shrubs being rich in 



