EXPOSURE TO LIGHT 



241 



Fig. 225. - 

 from above. 



Dandelion viewed 

 The leaves form a 



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 



occur at a node, as illustrated in 



Figure 224. Iil this arrangement rosette and the lower leaves are 



the leaves are also so placed as to ™uch longer than the upper ones. 



i_ J 1- iu 1-j.j.i -ui After Stevens, 



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 Hght exposure is called a leaf mosaic, being so named 



Fig, 



226. — Nasturtiums showing mosaic 

 arrangement of leaves. 



