PALO VERUE. I 7 r 



joined together by a small piece which forms a collar about the 

 caulicle. 



From the physiological point of view the green branches of 

 plants in which the leaves are very much reduced, or absent, are 

 so much leaf surface, just as in the cactus, which is entirely 

 devoid of functional leaves, the green tissue supplies their want. 

 Therefore we ma}- regard the green bark of the palo verde as so 

 much leaf surface. A transverse section cut through one of 

 these twigs shows a highly organized system of green cells, 

 having an arrangement quite similar to that found in the leaves 

 of man}- plants exposed to strong sunlight, as they are in the 

 desert. The stomata are similar in type to those of many cacti, 

 being immersed below the surface, each at the bottom of a pit. 

 Although the twigs are slender and easily bent and so give readily 

 to the wind, they are nevertheless very strong and tough, almost 

 like strands of leather. This is accounted for by the presence 

 of so-called bast, which in this plant is peculiar in that it traverses 

 the pith longitudinally. This, however, is supplemented by sim- 

 ilar strands found in the rind. 



All three species are well worthy of cultivation, although T 

 believe that the long-leaved species is the only one which has 

 thus far been introduced into cultivation and is known throughout 

 most of the warm regions of the earth. 



The Classificatiox of Cactaceae. — The grouping of the 

 species of cactuses into genera has been one of the most difficult 

 problems of systematic botany, because this classification has 

 necessarily been based largely on the plant body alone, inasmuch 

 as the flowers and fruits of many of the species have either been 

 unknown to botanists or imperfectly understood. It has been 

 known that certain features of structure of the plant body are 

 associated with characters of the flowers and fruits, and that if 

 these were known in all the species, a truly scientific grouping 

 could be established. As everyone who has grown cactuses 

 knows, many plants will remain alive in collections for years 

 without making an}- considerable growth and without flowering ; 



