312 PLANT STRUCTURES 



if narrow leaves were associated with few rows there woiiM 

 be waste of space. 



It is very common to observe the lower leaves of a sliin 

 long-petioled, those above short-petioled, and so on until 

 the uppermost have sessile blades, thus thrusting the blades 

 of lower leaves beyond the shadow of the upper leaves. 

 There may also be a gradual change in the size and direc- 

 tion of the leaves, the lower ones being relatively large and 

 horizontal, and the upper ones gradually smaller and more 

 directed upward. In the case of branched (compound) 

 leaves the reduction in the size of the upper leaves is not 

 so necessary, as the light strikes between the upper leaflets 

 and reaches those below. 



On stems exposed to light only or chiefly on one side, 

 the leaf blades are thrown to the lighted side in a variety 

 of ways. In ivies, many prostrate stems, horizontal branches 

 of trees, etc., the leaves brought to the lighted side are 

 observed to form regular mosaics, each leaf interfering 

 with its neighbor as little as possible. 



There is often need of protection against too intense 

 light, against chill, against rain, etc., which is provided 

 for in a great variety of ways. Coverings of hairs or scales, 

 the profile position, the temporary shifting of position, 

 rolling up or folding, reduction in size, etc., are some of 

 the common methods of protection. 



175. Shoots. — The stem is an organ which is mostly 

 related to the leaves it bears, the stem with its leaves being 

 the slioot. In the foliage-bearing stems the leaves must be 

 displayed to the light and air. Such stems may be sub- 

 terranean, prostrate, floating, climbing, or erect, and all of 

 these positions have their advantages and disadvantages, 

 the erect type being the most favorable for foliage display. 



In stems which bear scale leaves no light relation is 

 necessary, so that such shoots may be and often are sub- 

 terranean, and the leaves may overlap, as in scaly buds 

 and bulbs. The subterranean position is very favorable 



