254 



THE PI.ANT WORLD 



resin, or other water-proof material, or furnished with a volatile oil. — One 

 of the more common and widespread examples of this large group 

 is the creosote bush (^Covillea tridentata) , which is a characteristic plant 

 of extensive desert areas of North America. The endurance of the leaves 

 varies with the locality and the conditions, but in some instances it is 

 known to produce two crops of leaves and flowers during the year. In 

 general it may be said that the reddish-brown resin secreted by the 

 leaves is most abundant in the drier periods immediatel}^ following the 

 rainy seasons. Covillea is not furnished with spines, but the resin is so 

 strong in odor and taste that it is but little attacked by animals. (PI. 35.) 



VI. Perennials with succulent stems or leaves, or with other special 

 devices for the storage of water. — This group includes a wide diversity of 

 morphological types, and the species showing greatest capacity for 

 storage of water are most abundant in the regions in which the rainfall 

 occurs within a comparatively brief period . The total surface 

 exposed from which transpiration may take place is generally 

 comparatively small in proportion to the volume of the body 

 of the plant. The reduction of the shoot and the adaptation 

 of organs to water storage show the greatest diversity in 

 different species. 



In Cereus greggii the main root and the base of the shoot 

 appear to be converted into a storage organ containing a 

 large amount of water, while the shoot shows an extreme 

 reduction . The tuberous underground portions of this plant 

 are beet-shaped, a foot or two in length, and may weigh as 

 much as ten or fifteen pounds. This means that an amount 

 of water sufficient to meet the ordinary needs of the plant 

 for a year is held in reserve. (Fig. 5.) 



Ibervillea so?iorae, the guarequi of the Mexicans, is an 

 example of a plant that has converted the basal perennial 

 portion of the stem into a storage organ which holds a 

 supply of water sufficient to keep the plant alive for years. 

 The large compressed tubers which lie on the surface of the 

 soil may attain a diameter of 40 cm across and half that 

 amount vertically. The woody pulp contains in addition to 

 water, reserve food-material and some poisonous substance, 

 the nature of which has not yet been ascertained. It is 

 ^u.from'-rufs'o^^ evident, however, that it is sufficiently potent to secure the 

 va"ld^n the^New tubers agaiust the attacks of grazing animals. The tubers 

 Garden.^°'*°''^^^ ^^Y ^c sccu lying ou the sand under arborescent opuntias 

 and acacias during the dry season, and quickly send up the characteristic 

 leafy stems of the Cucurbitaceae upon the approach of rains. Seeds are 

 produced within two months from the beginning of activity and the 



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