70 



General Botany 



light. This difference in color is accounted for in part by the 

 amount of chlorophyll near the surface and in part by a 

 slight difference in the color of the green pigment in the chloro- 

 plasts. In a few shade plants the depth of the green color is 

 increased by the presence of chloroplasts in the epidermal cells. 

 Shade plants are not subjected to drying, as are plants growing in 

 exposed situations, and, generally speaking, their leaves are broad 

 and thin. The leaves of these plants differ further from the or- 

 dinary leaf in that the cuticle is less developed, the mesophyll 

 is composed almost entirely of spongy tissue, and usually stomata 

 are present on both surfaces of the leaf. 



Submerged leaves. Every one who has gone fishing or rowing 

 knows that a great deal of sunhght is reflected from the surface 

 of water. A smooth water surface reflects about one fourth, and 

 a rough water surface about one half, of the hght that falls on it. 



This means that the amount of 

 hght that passes into the water 

 is reduced by the amount that is 

 reflected. The penetration of 

 the water by the sun's rays is 

 further interfered with by the 

 fine sediment that clouds our 

 ponds and lakes. Every one 

 who has dived and opened his 

 eyes under water knows that it 

 is dark at a comparatively shght 

 depth. Measurements have 

 shown that one half to three 

 fourths of all the light that en- 

 ters the water may be stopped in 

 the first three feet, depending 

 upon the amount of suspended 

 particles present. Hence sub- 

 merged plants always grow in 



Fig. 40. Pulvinus and section of pulvinus 

 from leaf of sensitive plant, both enlarged. 

 When the leaf is touched, the water in 

 the cells on the side A passes outward 

 into the intercellular spaces, causing the 

 cells partially to collapse. The pressure 

 of the cells on the side B then forces the 

 leaf downward. 



