538 MEASURES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 



little forward, so that each in front is partly covered over by the one immediately 

 behind it; then the four ribs or axes, beset with the tiny leaflets, move towards one 

 another like fingers which had been outspread and are now closed together; and 

 thirdly, the common leaf -stalk, bearing in front the four axes, droops downwards. 

 The leaflets of several species of wood sorrel which have clover-like or fan-like 

 leaves, and not pinnate leaves like the above-mentioned Oxalis sensitiva, also fold 

 their leaflets together when shaken by rain-drops. In these species of wood sorrel, 

 however, we have again a way of diverting water, essentially difiering from that 

 above described. The common leaf-stalks do not bend towards the ground, but 

 remain erect; on the other hand, the drooping leaflets fold slightly along the midrib, 

 each of them forming a shallow groove, and the rain water trickles on to the 

 delicate leaves, and then flows through these channels to the ground. (Of. fig. 90 ^ 

 the lowest leaf, whose three leafiets are beginning to droop and to fold.) 



From the above it is indeed evident that one benefit which the sensitive plants 

 obtain by the sudden folding together of their leaflets lies in the rapid diversion 

 of the falling rain -drops thereby rendered possible. By this we do not imply 

 that this is the only advantage which ensues from the movements described. It 

 frequently happens that dry, gusty winds and drifting sand and extraordinary 

 noon-tide heat cause the folding of the leaflets. In the last-mentioned instances 

 it is rather the danger of excessive transpiration which causes the plants to place 

 the broad surfaces of their leaflets vertically, and all observations go to show that 

 the leaflets can escape very various dangers by the assumption of the so-called 

 sleep position — in the clear night, the loss of heat by radiation towards the starry 

 sky; in the hot mid -day, drying up in consequence of rapid evaporation; in rainy 

 weather, the breaking up of the tender leaves and their inclination towards the 

 ground, as well as the collapse of the whole plant under the weight of the falling 

 drops in a sudden severe downpour of rain. It is possible that yet a fourth 

 advantage is obtained by these movements. Grazing animals which snifl' the 

 delicate leaves of the sensitive plants and disturb them with their mouths are 

 perhaps astonished and frightened at the sudden movements of the leaflets, and 

 abstain from eating these uncanny plants, especially when between the descending 

 leaflets pointed rigid spines become visible, as is especially the case in many 

 mimosas. 



It cannot be too often insisted that the same and similar contrivances, as well 

 as the same and similar processes, may have a very difierent significance according 

 as they occur in this or that plant, in this or that habitat, and under these or 

 those climatic conditions; just as, on the other hand, several advantages can be 

 simultaneously obtained by one and the same contrivance. Thus for instance, 

 the position which the leaves emerging from the buds in spring assume is very 

 instructive. When the vegetative activity is interrupted by a cold winter, and 

 when, moreover, the temperature occasionally in clear spring nights sinks below 

 zero, the surfaces of the leaves just escaping from the buds are regularly placed 

 vertically (cf. fig. 90). During the day excessive transpiration from the still thin- 



