22 The Plant World. 



from countries warmer than our own endure our summer ex- 

 tremes well, although they are subject to severe injury from low 

 winter temperatures and are occasionally killed. Thus far the 

 introduced species that do best under our conditions are those 

 native to regions having similar climatic conditions to our own. 

 Many bulbous plants such as Narcissus and Zephyranthes thrive 

 with us, growing actively during one season of the year and 

 resting over the other, notwithstanding the usual irrigation. It 

 is well known that many such species as these are indigenous 

 to semi-desert parts of Africa, both north and south, and hence 

 have long been accustomed to climatic conditions quite similar 

 to our own. They represent a type of desert plant, the 

 growth of which is analagous to that of our winter-growing herbs. 

 It is beyond the limits of this paper to discuss the effects of 

 various soil conditions on the growth of plants. Unlike extremes 

 of heat that are most pronounced in the instance of species na- 

 tive to cool regions, or those of cold which are commonly the de- 

 ciding factor with extra-tropical plants, soil conditions exert an 

 influence in one way or another on the growth of nearly all 

 species, although this is often scarcely apparent. Not infre- 

 quently the physical texture of the soil is such as to prohibit 

 or at least to make uncertain the successful growth of a consider- 

 able number of plants. A few examples will have to suffice here. 

 Eucalypts do best in sandy or gravelly soil, in fact, within rea- 

 sonable limits, the more sandy the better. Such soils are well 

 aerated, free from alkali, and do not become water-logged 

 notwithstanding heavy flooding or irrigating, all of which are 

 essential to success in growing these trees. Among our hardiest 

 eucalypts, as concerns resistance to heat, cold, and aridity, are 

 Eucalyptus rtidis, E. rostrata, and E. polyanthema. These, how- 

 ever, will not grow in the presence of any considerable alkali, 

 or in heavy clay or adobe soils, which matter becomes the more 

 pronounced with frequent or heavy irrigation. This explains 

 partly the failure of these plants to grow in numerous instances 

 in southern Arizona. This condition is apparent when scattered 

 individuals over a eucalypt plat, or frequently nearly all the 

 plants become yellow, cease growth, and finally die, while occa- 

 sional plants more fortunate as concerns soil or drainage condi- 

 tions continue with foliage dark green in color and vigorous in 

 growth. As a result of the flood waters from the heavy summer 



