Environment and Adaptation 201 



grown for ornament, and their real nature can be 

 seen by tracing the development of the leaves in 

 the seedling, which at first always show a feathery 

 lamina, which is gradually reduced in the later 

 leaves, until it quite disappears, and nothing is left 

 but a flattened leaf stalk. When these normally 

 xerophytic plants are abundantly watered, it is quite 

 common to find the leaves reverting to the feathery 

 form on the more vigorous young shoots. In other 

 cases like the prickly pear and the greenhouse 

 " smilax " the leaf-like organs are really flattened 

 branches. 



Water Storage. The storage of water is also an 

 important function in xerophytic plants, and there 

 are many types of storage organs. The root sys- 

 tem is also modified with reference to the water 

 supply and to the character of the aerial parts. In 

 desert plants the roots may be very long and capa- 

 ble of reaching down to the deep-seated layer of 

 water in the soil, or as has recently been shown by 

 Cannon in his studies on the roots of desert plants, 

 the root system may be very shallow, spreading ex- 

 tensively near the surface of the ground, where 

 advantage may be taken of brief showers which 

 wet only the superficial layers of the soil, and 

 quickly evaporate. The water is promptly absorbed 

 and conveyed to the aerial parts of the plant and 

 there stored away for future use. The great cacti 

 of the hot, arid regions of the Southwest are very 

 striking examples of this xerophytic type. 



