RELATION BETWEEN POSITION AND FORM OF GREEN LEAVES. 



411 



insertion upon the abbreviated axis. It is unavoidable that the narrower, 

 proximal halves of most of the leaves should be covered by the leaves above and 

 fail to receive sufficient light. But these covered portions are always destitute of 

 chlorophyll, and so have no need of direct sunlight. The distal halves, on the other 

 hand, which display green tissue, can by this arrangement be all well illumined 

 simultaneously by the sun. In many other instances the increase in length is only 

 found in the leaf-stalks of the lower leaves of the rosette. These increase in length, 

 that is to say, until the blades borne by them are moved out of the shadow of the 



Fig. 104. — FonnatioD of a Leaf-Mosaic by tlie lengthening (relative shortening) of the Leaf-stalks. 

 1 Small-leaved Balsam (Impatiens parvifiora). a Green Amaranth (Amarantus Elitum). s Thorn-apple {Datura Stramonium). 



leaves above. This is the case, for example, in the leaf-rosettes of Geranium 

 Pyrenaicv/m, represented in fig. 103 ^ and in the leaf -rosettes of the dainty little 

 bell-flower (Campanula pusilla, fig. 103^) growing on the debris-covered slopes of 

 the sub-alpine regions. In these bell-flowers the great diflference in shape between 

 the rosette-leaves and those clothing the flower-stalk is worthy of remark. The 

 latter, which spring at an acute angle from the stem, are narrowly lanceolate, and 

 have very short stalks, while the lower rosette-leaves, extended flatly over the soil, 

 have long stalks, and possess a broad, ovate blade. It is no disadvantage to the 

 leaf -stalks, which have no chlorophyll, if they are placed in the shade. But by this 

 arrangement all the broad, green leaf -blades are well illumined, and this applies 

 also to the more loosely-arranged, upwardly-directed, narrow leaves of the stem. 



