EXPOSURE TO LIGHT 



241 



Figure 222. Many trees as well as many herbaceous plants, 

 such as Cotton, Clover, Alfalfa, Tomatoes, Potatoes, Buckwheat, 

 and Flax, have the alternate arrangement of leaves. In the 

 opposite arrangement two leaves appear at each node on opposite 

 sides of the stem, and neighboring 

 pairs are set more or less at right 

 angles to each other, so that as one 

 looks down from above each pair 

 of leaves alternates in position with 

 the pair above and with the pair 

 below it as shown in Figure 223. 

 The opposite arrangement is also 

 common among both woody and 

 herbaceous plants. In the whorled 



arrangement more than two leaves FIQ ^ _ ^M viewed 

 occur at a node, as illustrated in f rom a b ov e. The leaves form a 

 Figure 224- In this arrangement rosette and the lower leaves are 

 the leaves are also so placed as to much lon g er than the u PP er ones - 

 shade each other as little as possible. 



In plants, like the Dandelion and Plantain, which have very short 

 stems bearing many leaves, the leaves form a mat, called a rosette, 



on the surface of the 

 ground. It is readily seen 

 that leaves so closely 

 crowded as they are in the 

 rosette must shade each 

 other considerably, but 

 they have the advantage 

 of being exposed less than 

 those on elongated stems 

 to the loss of water by 

 transpiration. In the 

 rosette much shading is 

 eliminated by a difference 

 in length of petioles, for the outer and under leaves of the rosette 

 have longer petioles which push their blades beyond those of the 

 upper leaves, and in this way they escape the shade of the leaves 

 above. This feature is noticeable in the rosette of the Dandelion 

 shown in Figure 225. Another arrangement of leaves which is 

 favorable to light exposure is called a leaf mosaic, being so named 



FIG. 226. Nasturtiums showing mosaic 

 arrangement of leaves. 



