626 EEV. G. HENSLOW ON VERNATION AND FOLIAGE 



tion; nor has any one noticed the special behaviour of deve- 

 loping leaves and stipules, as well as of the axes and petioles 

 which support them. If, however, we keep in view the two 

 assumed objects — first, the securing an erect or dependent 

 position of the blades so as to place their surfaces in a plane at 

 right angles to the surface of the earth ; and, secondly, the pro- 

 tection of the upper surfaces, — it will be seen how various are the 

 efforts of nature to secure these two ends during the period of 

 development, and while the young leaves and shoots are succulent 

 and delicate, such being only too readily and often severely 

 injured by the cool nights, sharp frosts, and cutting winds of 

 spring, at the very time when the buds are expanding. The 

 various methods of protection are more perfect than in sleep- 

 ing plants, inasmuch as the young leaves are more delicate than 

 when adult. Testing the effects of desiccation by the heat of the 

 sun, I found that when young clover-leaves naturally conduplicate 

 were forcibly spread out with the upper surfaces exposed to the 

 sun for twenty minutes, the edges soon curled inwards, and they 

 lost 37 - 2 per cent, of weight. Those spread out with their under 

 surfaces exposed remained flat, but lost 43*2 per cent, of weight 

 in the same time, i. e. rather more than the preceding. In all 

 experiments with clover-leaves exposed at night, I found the dif- 

 ferences were not so pronounced as in many other plants. 



In observing the effects of frost upon delicate leaves one sees 

 that they are especially injured along the margins ; and, more- 

 over, where so affected, they are more or less dried up. The 

 first fact gives a significance to the revolute and involute kinds 

 of vernation, in which the margins are rolled outwards and 

 inwards respectively. This led me to suspect that whenever 

 leaves suffered from radiation, not only was there a reduction of 

 temperature, but a loss of moisture would seem to accom- 

 pany the loss of heat. A similar loss of moisture would occur 

 by desiccation, in consequence of cool dry winds, and must be 

 equally guarded against. If this were so, then the balance would 

 indicate the loss. With this object in view, I weighed a number 

 of undeveloped leaves together of several plants, and made two 

 groups of each, selecting leaves as nearly like as possible for each 

 group of the same plant. I then, following Mr. Darwin'a 

 method, fixed all of one group with their upper surfaces exposed 

 upwards with card-clips, such as are used by entomologists for 

 spreading out the wings of butterflies ; the other group of leaves 



