84 EDITH SCHWARTZ CLEMENTS 



As is well known, shade-leaves are characteristically large and thin 

 as compared with sun-leaves. With a few exceptions this holds for 

 the group, the reduction in thickness affecting both the palisade and 

 sponge tissues. The increase in the leaf of Fragaria may be due 

 to the fact that the leaf is already as thin as is consistent with the 

 proper performance of the physiological functions of the species. 

 The difference between the two deeply shaded forms themselves, the 

 one with one-tenth as much light as the other, is in size of leaf, the 

 extremely shaded plant being able to develop no further than the 

 seedling stage. It is probable that beyond certain limits, changes 

 in physical factors do not produce corresponding changes in the 

 histology of the leaf. This is shown by all of the seedling forms 

 growing in intense shade. The more shaded leaf of Polemonium is 

 neither thinner nor thicker, as the light differences are not very 

 great. The palisade cells, however, are more loosely arranged, in 

 response to the decreased light or increased humidity or both, as 

 both have that effect. Lactuca ludoviciana and Saxifraga inter- 

 rupta seem to be relatively stable species. The only change in the 

 former is a thinner cuticle, while the latter remains unchanged. 

 The increase in sponge in Edwinia may be only apparent since there 

 is no distinct line between the palisade and the sponge tissues, and 

 one had to be arbitrarily drawn for purposes of comparison. 



As shown by the table, reduced light, besides decreasing the pal- 

 isade and sponge tissues in amount, shortens and broadens the pal- 

 isade cells and extends the sponge cells in a horizontal direction. 

 The extreme of this tendency is to be seen in Sparganium which has 

 exactly reversed the long axis of the palisade cells. By this means 

 the chloroplasts are placed in the most favorable position to utilize 

 the weak light. It has been suggested that the impulse to the 

 change in shape of the cells and the consequent thinning of the leaf 

 comes from the mobility of the chloroplasts. In the majority of 

 the shade-leaves the stretching of the palisade cells is more apparent 

 at the top of the cell and the so-called " funnel " form is the result. 

 This form of cell is well adapted to place the chloroplasts in a favor- 

 able position in respect to light and in consequence of the resulting 

 spaces between, to allow the penetration of light to the interior of 

 the leaf as well. The loose arrangement of the prolate palisade 

 cells also serves this end, although probably also bound up with 

 the necessity for an increase in transpiration. 



