348 PLANT-BREEDING 



all. All their visible marks point to a reduction in the 

 use of water, the evaporating surface being as thoroughly 

 reduced as possible. Under the ground, the development 

 of their root-system is quite the reverse. The roots are 

 long and widely branching, penetrating to a considerable 

 depth, thus enabling the plants to procure the necessary 

 water. 



. The two other types of desert plants are the cacti and the 

 annual weeds. The roots of the cacti are spread laterally, 

 instead of growing to any considerable depth. They may 

 be said to drain the surface all around the plants. They ab- 

 sorb the rainwater at the periods of those short but heavy 

 showers, which for a short time moisten the soil and stimu- 

 late vegetation. This water is brought into their fleshy stems 

 and stored up, that it may be used afterward during the 

 long rainless seasons. During rainy weather the stems 

 of the cacti are seen to swell, shrinking again in the succeed- 

 ing periods of drought. The annual weeds of the desert 

 are distinguished by the shortness of their life- time, this 

 being limited to the few weeks of rainy weather in the 

 spring. As soon as the summer begins and the soil 

 is drying, their life-cycle is completed, and only the dead 

 stems and the numerous seeds remind the visitor of the 

 profusion of flowers which have vanished. 



In the mind of a botanist strolling on the arid soils 

 amidst these most strange and astonishing forms of vegeta- 

 tion, the question necessarily arises: Are all these species 

 natives of the desert, and have they acquired their special 

 characters under the influence of the long periods of dryness 

 and the insufficiency of standing water in the soil? Is it 

 the desert which has made them such as they are now, 

 apparently admirably suited for these extreme life-conditions ? 

 Or, on the other hand, are they perhaps only a selected 

 few from among the widely differentiated forms, which 



