90 BOTANICAL GAZETTE [February 



biennis, for example, when growing in a dry, sterile soil and exposed 

 to strong wind and maximum sunlight, is found to have smaller 

 and thicker leaves, more perfectly developed palisade parenchyma, 

 a more hairy and a more densely cutinized epidermis, and, in 

 general, a more xerophytic structure than the same species growing 

 under more favorable conditions. 



A similar structural difference is apparent in summer and 

 winter leaves, or in stem and rosette leaves. Winter leaves, as 

 the name implies, exist during the winter, which, in our latitude, 

 is the most unfavorable season of the year. During the winter 

 transpiration becomes relatively excessive because of the reduced 

 rate of absorption, and plants are thus put to the severest test. 

 Sometimes for days at a time the ground is frozen and absorption is 

 practically zero; while during the warmest part of the day con- 

 siderable transpiration may take place. The plant is thus exposed 

 to the danger of desiccation. Moreover, during the night the most 

 exposed leaves may freeze hard. Toward noon of the following 

 day they may thaw out, presenting a wilted condition as if killed 

 by scalding. However, it is surprising how quickly such leaves 

 will revive as conditions again become more favorable. No sooner 

 is the absorption of soil water resumed than the leaves once more 

 become turgid and resume their wonted appearance, apparently 

 none the worse for the ordeal through which they have passed. 



Since winter leaves are exposed to such severe conditions, it 

 would be natural to suppose that they must be quite xerophytic 

 in structure. While this is true to a certain extent, in some respects 

 they are less exposed to unfavorable conditions than stem leaves. 

 This is especially true of winter leaves occurring in rosettes. In 

 rosettes the internodes are extremely short and the leaves thus 

 become closely crowded and overlapping. Since epinasty pre- 

 vails during the winter, these overlapping leaves lie almost flat on 

 the ground, thus affording maximum protection for each other from 

 sudden changes of temperature, as well as from high winds and 

 excessive transpiration. It is seldom that winter leaves die as the 

 direct result of freezing, and when it is borne in mind that such 

 leaves have a low water content and a high osmotic pressure, thus 

 insuring easier absorption of soil water, the protection would seem 



