THE LEAF. 9 



them in every direction, giving each its share of room and ex- 

 posure to light and air, and to bear the flowers and seeds. 



" The stem, in fact, is the agency by which the work of indi- 

 vidual leaves is combined and concentrated for the general benefit 

 of the plant. Each separate leaf, like each separate cell, has a 

 life of its own, and to some extent is independent of every other 

 leaf ; but if they are to be of any use to the plant as a whole, there 

 must be cooperation." (Master's Plant Life.) 



The explanation for the ascent and movement of the "sap" in 

 plants is by no means simple and easy. The swaying of the 

 stems, branches, and leaves by the wind renders some assistance. 

 The chemical changes going on within the plant cause some 

 movements of the liquid materials. The evaporation from the 

 leaves helps "draw" the water and gases from below to ascend 

 and fill the spaces which would otherwise be vacant. " There is 

 no continuous tube or set of tubes, and there is no fluid of uni- 

 formly the same composition throughout. Near the root the 

 juice of the plant has one composition, near the leaf another. 

 The word 'sap,' then, though convenient, must not be used or 

 conceived of as indicating the existence of a current absolutely 

 fixed in its direction or uniform in its composition." (Master's 

 Plant Life.) 



The Leaf. Springing from the superficial part of each node, 

 and generally completely surrounding the culm, appears a leaf, 

 the sheath or lower part of which is generally convolute or wrapped 

 around the culm. The leaves are two-ranked or distichous, and 

 are so placed that each leaf is a little above or below any special 

 one selected and exactly half way around the stem, where the 

 blade spreads away from the stem. L T sually there is one leaf 

 at a node, but in Cynodon Dactylon (Bermuda grass), Sporobolus 

 arenariiis, and a few others, there are apparently two or three at 

 a node distichously placed above each other. 



The Sheaths of the leaves are usually spoken of as split on 



