88 PLANTS 



they are found to be crowded on the vertical walls of the cells, 

 while in subdued light they are ranged on the horizontal 

 walls, thus exposing themselves broadside to the light. In this 

 way the chloroplasts to a considerable degree control the 

 light relation of the plant. 



207. Plants might be classified with reference to the light 

 conditions of their habitat. Many species grow only in shaded 

 situations, while others seek the brightest light. Under the 

 vertical rays of a tropical sun the light is much more intense than 

 it is in higher latitudes. Consequently it penetrates the 

 foliage of the taller forest vegetation with sufficient intensity 

 to permit also of a vigorous undergrowth. The result is that 

 other things being equal the intensity of the light in the tropics 

 permits a denser growth of vegetation than could exist in higher 

 latitudes. 



208. The deep sea is known to be practically destitute of 

 vegetation, although the conditions, except for darkness, are 

 probably favorable. Along shore and on the surface of the sea 

 there is an abundance of green and brown sea-weed vegetation, 

 and farther down, on comparatively shallow bottom, the red 

 sea-weeds are found. It has been suggested that the red and 

 brown pigments of the red and brown sea-weeds have some 

 significance with reference to the light relation. 



Soil 



209. Every farmer is familiar with the fact that the dis- 

 tribution of plants is largely determined by the nature of the 

 mineral constituents of the soil. Thus limestone regions are 

 better adapted for the cultivation of certain crops than are soils 

 derived from sandstones or shales. So also other plants do well 

 only on sands or clays. Analysis of the soils, however, shows 

 that all the mineral substances necessary to any plant are 

 present in sufficient quantities in any soil. It is also known that 



