THE AMERICAN BOTANIST. 67 



seasons are so marked as in Ohio one takes it as quite 

 natural that there should be a resting period in the winter. 

 But manj'- plants pass into a period of rest even if growing 

 in an environment perennially favorable. Thus it is very 

 common for complete defoliation to take place in many 

 plants of the tropics. It is said that there are nearly two 

 hundred species in Ceylon which become leafless at differ- 

 ent times of the year. The statement is made that there 

 is not a month when all the trees are in full leaf. It is evi- 

 dent, therefore, that in many cases the period of rest and 

 the deciduous habit are independent of climatic conditions 

 no difference how the character was originally acc^uired. 

 In our own plants the influence ot cold is no doubt pre- 

 dominant. The injuries of winter are not only due to the 

 direct effect of cold upon the protoplasm, but also to the 

 loss of water. With the approach of autumn, the chilled 

 roots are unable to supply the necessary amount of water 

 for the transpiration going on above ; consequently there 

 is a great advantage in reducing the transpiration surface 

 b^' shedding the leaves. Thus we might say that the cast- 

 ing of the leaves is an adjustment to a more limited water 

 supply. Plants may of course go into a period of rest 

 without shedding their leaves, as in our common conifers. 

 In most cases, however, there is a great change in the 

 bod}' of the plant or some of its parts to prepare for the 

 severe conditions. The annuals die completely and the 

 onl}^ resting period is in the seed. The biennials usuall}' 

 grow but little after the cold becomes severe. The greater 

 number of geophilous plants die to the ground. The 

 woody plants and a few herbs have mostly learned to en- 

 dure the winter by especially developed stems, the leaf 

 which represents the active transpiring and food manu- 

 facturing organ being usually shed. 



The methods by which the leaves are separated from 

 the stem are various. Some plants like the hemlock shed 

 them after they are several years old. Others like the pines 

 get rid of the foliage leaves by pruning off dwarf branches 

 of a certain age. Some like the bald cypress and tamarix 



