212 



LEAVES 



whorled leaves that the advantage of petioles which are 

 graded in length is most apparent. This is especially 

 true of plants which, like the dandelion or the plantain, 

 have very short stems and their leaves in whorls. (See 

 Figure 75.) Such an arrangement of leaves as these plants 

 have is called a rosette, a word which describes their form 



very well. The outer 

 and under leaves of a 

 rosette have the longest 

 £7S*£^ v /JU\k petioles, and they de- 



crease in length toward 

 the top and center. 

 Such plants lack the ad- 



Fig. 75. — A dandelion showing the rosette vantage of long Stems 

 type of leaf arrangement. . , , ,. 



whereby to reach the 

 light, but they certainly take advantage of all the light 

 that reaches them. Also this form gives excellent protec- 

 tion against that deadliest of all plant enemies — the loss 

 of too much water. Another arrangement of leaves which 

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

 so named from the fact that the edges of the leaves as 

 viewed from above fit together like the little tiles of a real 

 mosaic. Leaf mosaics are common among vines. (See 

 Figure 76) 



D. Leaf Surfaces. — Leaf surfaces are usually either 

 smooth or hairy. Smooth plant . surfaces are sometimes 

 covered by a film known as the bloom; such surfaces are 

 said to be glaucous. The bloom is a thin coating of wax 

 which is secreted by the epidermal cells. It retards evapo- 

 ration. Surfaces from which it has been rubbed off have 

 been observed to lose water much more rapidly than before. 



