56 THE BOOK OF NATURE STUDY 



The movements of the wood-sorrel leaves have been thoroughly 

 investigated by Darwin and other botanists. Darwin also describes 

 in detail the sleep movements common to many Lupines. " The 

 shorter leaflets/' he says, " which generally face the centre of 

 the plant, sink at night, whilst the longer ones on the opposite 

 side rise, the intermediate and lateral ones merely twisting on 

 their own axis." Other plants with sleep movements are 

 Melilot, Clover, Lotus, Mimosa. In fact, it may almost be 

 said that sleep movements are specially characteristic of the 

 Leguminosae. 



Pfeffer has devised methods by which leaves register their 

 own movements. The records show not only the sleep move- 

 ments, but the automatic movements which often accompany 

 them. He found that under constant conditions the sleep 

 movements disappeared in from three to five days, but automatic 

 movements of much shorter rhythm continued independently 

 of the sleep movements. 



(/) The difference in cotyledons. To find cotyledons of 

 shrubs and trees, look in the humus of woods, under and around 

 the tree or shrub, in the months of April and May. Very often 

 the cotyledons will be seen pushing their way through the dead 

 leaves ; it will often be possible to find some when at the first 

 glance there may appear to be none, by scraping away some of 

 the upper layers of the leaves. 



THE WORK OF THE STEM. The main work of the stem is to 

 minister to the leaves. 



1. The stem bears the leaves in such a way that they can all 

 get light which, as we have seen, is necessary for the performance 

 of some of their functions. 



2. It conducts food to the leaves. The woody tissue in the 

 root is continuous with that of the stem. By root-pressure water 

 with mineral salts dissolved in it is forced into the wood of the 

 root, thence it travels the whole length of the stem, to the leaf- 

 stalk and veins of the leaf. 



3. The stem conducts food from the leaves to the root. The 

 starch made by the leaf is converted into sugar and passes, mostly 

 by the outside cells of the stem, down to the root, where it is stored 



