20 



The Plant World. 



easily raised plants of moister regions, are very uncertain, some 

 varieties accommodating themselves fairly well to the desert 

 air, while others fail altogether. Sweet peas have to be sown in 

 fall or winter to get the best results, as they succumb to the hot, 

 dry air of summer. Cobaea scandens does well in a greenhouse, 

 but seems struck with death when brought into the outdoor air. 

 Madeira vines form small leaves, and make an unsatisfactory 

 growth. Cannas and gladioli, which grow side by side in the 

 Ivast, part company here, the former making a good growth in 

 Arizona gardens, the latter failing altogether. Those who have 

 handled roses for a period of years have learned what varieties 

 may be expected to do well, and what ones may be counted out, 

 and so on through a long list of plants which, by knowledge 

 gained in the costly school of experience, are coming to be de- 

 pended upon, or are being rejected one after another, as they 

 are found to be unsuited to the environment into which they 

 have been brought. Thus, in a purely empirical way, it has been 

 found that many plants successfully cultivated in regions of 

 greater atmospheric humidity make an entirely normal growth 

 in the dry air of the desert, if their roots are well supplied with 

 water, but that others, however well cared for in this respect, 

 either fail entirely, or come short of making a healthy growth, 

 and this is especially true in the summer months when desert 

 conditions are most pronounced. 



The general fact thus broadly stated presents an inviting 

 but intricate problem to the plant physiologist, and from the 

 widely different behavior in the desert on the part of plants which 

 are successfully grown together in regions of greater rainfall, 

 it is evident that each plant has its own peculiar relations to 

 its environment, which must be investigated separately before 

 general conclusions can be safely drawn. It has been suggested 

 that plants which fail to withstand the severe conditions of an 

 Arizona summer are unable, no matter how well supplied with 

 water, to absorb and pass on to the leaves enough to make up 

 for their increased rate of transpiration, but where is the real 

 source of the difficulty ? Are the leaves structurally unfit to 

 control transpiration? Are the roots of one variety of geranium, 

 say, less capable of absorption in large quantities than are those 

 of another? Is the conducting system at fault? Or may it be 



