SENSITIVE PLANTS 



The photograph shows a cultivated species 

 of oxahs, whose relative, the Common Wood- 

 sorrel (Oxalis acetosel/a), is familiar during early 

 spring in woods almost everywhere in the British 

 Isles. Its characteristic clover-like leaves (of a 

 pleasant acid taste) and white purple-veined 

 flowers readily distinguish it. The species shown 

 here was photographed just before dusk, and it 

 will be seen that the three leaflets of each leaf have 

 turned down towards their stalks, and are now 

 somewhat like partially closed umbrellas ; later, 

 when darkness comes, they will close still more, 

 and become huddled together closely round their 

 stalks. This same characteristic of drooping leaf- 

 lets at night may also be observed in the common 

 British wood-sorrel. When daylight appears the 

 leaflets once more spread themselves out to the 

 sunlight. It becomes quite apparent, therefore, 

 that they have been to sleep, for each night the 

 leaflets fold together, and each morning they 

 open out again. 



Here, then, we have the beginning of sensi- 

 tiveness in leaves. The leaflets of the various 

 species of oxalis are usually ver}^ thin and of frail 

 texture, and their function is the same as that of 

 the leaves of other plants, viz., to spread their 

 tissues out to the sunlight, and under its influence 

 to absorb gaseous food from the atmosphere. At 

 night, when sunlight ceases, the leaves can no 

 longer carry on their feeding process, for sunhght 

 is essential. 



Seeing, then, that the leaves serve no purpose 



63 



