10 The Slruciurc of Leaves. 



direction, Tims it is that incisions are formed on the margins of 

 leaves. 



So much is the shape of the leaf varied, that it has been said 

 and from exjjerience 1 should say with truth, that no tv\o leaves, 

 "•ailiered from separate specimens, even of the same plant, can be 

 found to coincide exactly. But besides this infinity of shape 

 there is also a great variety of modifications. Indeed every part 

 of the plant that does not form the stem, is a modification of the 

 leaf — a fact which was first shown by J. J. Rousseau. Instances 

 of such modification are " the flower," the " organs contained by 

 the flower," the " pitcher plant," the "fly -catcher," the "bulb" of 

 the tulip, and many more which we cannot take into consideration 

 to-day. 



We must now consider the arrangement of leaves. It is not 

 to be supposed that leaves are placed upon the stalk at random, 

 for not only does common sense show this to be impossible, since 

 they would then materially interfere with one another; but, further 

 than this, the position of every leaf can be accounted for. 



It must have struck everylaody how strange it is that out of the 

 multitude of leaves on a tree, not one of which can flourish without 

 sunlight and free space around it, there is not one that impedes the 

 growth of another. Sorely puzzled should we be if required to 

 devise a plan of arrangement to satisfy these requirements. Nature, 

 however, manages it easily enough. Leaves are arranged in spirals, 

 so that they are always separate from one another, and have free 

 space around them. There is, of course, great variety as to the 

 number of leaves that complete a spiral round the stem, but each 

 order of plants is consistent with itself. 



Here, then, we have followed the " leaf" from its earliest stage 

 through its formation ; we have considered its shape and its arrange- 

 ment on the stem ; but, further, we must enquire what is the use of 

 leaves at all r Why should some be green and others (as those of 

 the flower) of most varied hues ? Why do they change their colour ? 

 Wh}'^ do they die } Let us now consider some of these questions ; 

 and first, what is the use of leaves? The best way, perhaps, of 

 showing this is by calling attention to the creeper, which so fre- 

 quently covers windows and arbours in summer, known by the 

 name of the "Canariensis." Sometimes the stem of a canariensis 

 gets broken quite at the bottom, just above the root. What is the 

 result ? — the plant does not die, though perhaps it does not flourish 

 so luxuriantly as before, perhaps it ceases to throw out any fresh 

 shoots, perhaps some of the leaves wither, but still the jylant lives 

 without the root. Now what does this prove ? — why that the leaves 

 help to nourish the parent plant. And indeed they do, and that 

 in many ways, but especially in supplying an element from the 



