ECOLOGY 



83 



parts of the earth vegetation flourishes, provided there is a 

 suflicient supply of moisture. In fact, it is in equatorial regions 

 that vegetation grows most luxuriantly. 



197. Between the two extremes of latitude, with the corre- 

 sponding extremes of cold and heat and absence and profusion 

 of vegetation, there are regions which present an alternation of 

 conditions with respect to temperature, from winter to summer. 



Fig. 41. — Same as the preceding figure but photographed on the preceding 

 day. The plant has the xerophytic habit of curhng up during dry weather as in 

 this figure. In wet weather the leaves expand. 



The change of seasons permits only of intermittent periods of 

 growth, and this has affected most of the species indigenous 

 to such regions to such an extent that the life processes succeed 

 each other in a rhythmical manner, even though the conditions 

 are temporarily altered. The lower orders of plants respond 

 more directly to actual changes of the conditions, and they vary 

 in character directly as the conditions vary, but the higher 



