ADAPTATION TO LIGHT 



179 



the relative size and vigor of the sun and shade plants must be 

 taken into account. The relation between surface and thickness 

 is shown by the following species, in all of which the leaf is larger 

 in the shade than in the sun. In Allionia linearis, the thickness 

 of the shade leaf is one fourth that of the sun leaf, i.e., the ratio is 

 3 : 12, and in Capnoides aureum 6 : 12. The ratio in Thalictrum sparsi- 

 Jiorimi is 9:12, and in Machcer anther a aspera 11:12. The ratio of 

 thickness of the shade and sun forms of Bursa hursa-pastoris is 



Fig. 61. -Sun and shade torm of Senecio taraxacoides. The leaves are 

 larger, thinner, and smoother in the shade plant, and usually more 

 entire. 



14:12, but the greater thickness of the shade leaf is explained by 

 the fact that this plant is ten times larger than the sun form. 

 Upon the basis of size, the thickness of the shade leaf is scarcely 

 one ninth that of the sun leaf. Some species show no change in 

 thickness, and but little in size or outline. This is doubtless to be 

 explained by the fact that the form is so fixed in the sun plant that 

 the decrease in light intensity has little or no effect upon it. 



197. The form of stems. Shade ecads are regularly taller and 

 often more branched than the corresponding sun form. In general, 

 this statement is true of all shade plants as compared with sun 



