160 



CASSELL'S POPULAE G-ARDENING. 



The " Insertion " of 

 Leaves. — The mode in 

 which leaves spring from 

 the stem is also a matter 

 of primary importance to 

 the botanist, and it has 

 also a distinct phj-siolo- 

 gical significance. Leaves 

 are said to he " opposite " 

 when they spring from 

 opposite sides of the stem 

 at the same level; " verti- 

 cillate," or " whorled," 

 when a ring of leaves start 

 from one point, so as to 

 form a " whorl" (Fig. 46) : 

 and " alternate " when 

 they spring from different 

 sides of the stem, one 

 above another at different 

 levels, that is, as in the 

 Elm or Peach (Fig. 47). 

 So, too, the leaves are in 

 some cases very densely 

 packed, as in a Plantain or 

 Cabbage, when the " in- 

 tervals " or spaces be- 

 tween the nodes are un- 

 developed or loosely 

 scattered ; usually they 

 are more or less arranged 

 in spiral lines. 



Adaptation to Pur- 

 pose. — ^It would not be 

 needful to mention these 

 details here were it not 

 for the circumstance that 

 the form and disposition 

 of the leaves have a dis- 

 tinct connection with the 

 work they have to do, with 

 the necessity for literally 

 " catching sunbeams," or 

 availing themselves of the 

 fresh air, and, moreover, 

 that they are often the 

 result not only of adapta- 

 tion to particular pur- 

 poses, but the outcome of 

 a compromise in virtue of 

 which each leaf does its 

 best for itself, whilst re- 

 specting the rights of its 

 neighbour. All these 

 variations in size, that 



Fig. 47.- 



-Branch of Peach, sliowing 

 simple leaves. 



alternate 



amazing diversity of form 

 and appearance, all those 

 twists and turns, those 

 upward movements, those 

 sidelong motions, have 

 each a distinct purpose 

 and significance. They 

 are not mere caprices, but 

 they represent the stiug- 

 gies of the plant for light, 

 air, and supremacy, its 

 conflicts and rivalries with 

 its neighbours ; or, if in 

 some cases this adaptation 

 of means to ends is not 

 clearly marked out under 

 existing circumstances, we 

 may be sure that it was 

 so once, and that what 

 we see now are historical 

 survivals from a period 

 when these matters had a 

 real significance. 



Internal Structure. 



— In like manner, the in- 

 ternal microscopic struc- 

 ture of the leaves shows a 

 similar adaptation to pur- 

 pose, and it is often easy 

 to see a co-relation be- 

 tw'een the structm^e of the 

 leaf, and the disposition 

 of its elements, wdth the 

 conditions under w^hich 

 it grows. The leaf of 

 a plant growing in water 

 (Fig. 48) is very diiferent 

 in • structure from that 

 of an evergreen exposed 

 to full sunshine on a 

 hill-side, or a succulent 

 plant on a rock. The 

 elements are, indeed, 

 much the same, but they 

 are differently arranged, 

 and their degree of de- 

 velopment is modified ac- 

 cording to circumstances. 

 For these reasons we can 

 here only give a general 

 statement with reference 

 to leaf-structure, referring 

 the reader to ordinaiy 

 text-books for fuller in- 

 formation, and earnestly 



