538 



ECOLOGY 



from rather than approach sphericity. However, cultures of isolated palisade cells 

 show an approach toward sphericity (figs. 774, 775), as though lateral pressure 

 had been removed suggesting the possibility that the close arrangement of the 

 palisade cells in the leaf prevents the assumption of the spherical form. In the 

 same connection it is of interest to note that when shade monocotyls are exposed to 

 intense light or to strong transpiration, the cells which usually are palisade-like and 

 elongated parallel to the surface tend to approach sphericity. Furthermore, leaves 

 attacked by parasitic fungi show increased palisade 

 development, suggesting the possibility that with the 

 introduction of fungi the osmotic pressure of plant 

 cells is increased (see p. 746). Where an increase of 

 light or of carbon dioxid increases the synthesis of 

 carbohydrates, there results an increase of the 

 osmotically active substances (particularly sugars) 

 within the cell and hence an increase of the cell 

 turgor, suggesting that in the last analysis the light 

 theory and the transpiration theory of palisade de- 

 velopment may be essentially identical. 



Causes of variation in the position of chloren- 

 chyma. — The most significant fact regarding varia- 

 tion in the position of chlorenchyma is the presence 

 of epidermal chlorophyll in submersed hydrophytes 

 and in some shade plants, and its absence in other 

 plants, except in the guard cells of stomata. No 

 adequate cause for such variation is known, possibly 

 because the problem has not been seriously attacked. 

 That an external cause is probable, at least at times, 

 is evident from the fact that various amphibious 

 plants (as species of Ranunculus) exhibit epidermal 

 chlorophyll in the water, but not in the air. Per- 

 haps light is a factor, since epidermal chlorophyll is confined essentially to habitats 

 where the light is of low intensity; deep-lying chlorophyll seems to be impossible 

 under such conditions. On the other hand, if intense light is deleterious, the lack 

 of epidermal chlorophyll in sun plants may be accounted for, though its absence 

 in most shade plants would remain unexplained. 



Figs. 774, 775. — Isolated 

 palisade cells of Lamium pur- 

 pureum: 774, two cells which 

 have been in a nutrient solu- 

 tion for eight days; the lower 

 cell was injured upon removal 

 from the leaf, showing no fur- 

 ther growth; the upper cell 

 grew considerably, especially 

 in breadth; 775, two palisade 

 cells similarly treated; note 

 the great growth in breadth, 

 compared with the growth in 

 length ; highly magnified. — 

 From Haberlandt. 



The advantages of differentiation in chlorophyll-bearing tissues. — The 

 chief activity associated with chlorenchyma is the synthesis of carbo- 

 hydrates. Probably the palisade layers form the chief synthetic tissue, 

 the spongy tissue having more to do with aeration and hence with trans- 

 piration (p. 552). The paUsades are relatively more efficient in diffuse 

 light, because of their more favorable position, while in intense light the 

 efficiency of the sponge cells increases; in very intense light they may 

 surpass the paUsades in synthetic importance, because of the deleterious 

 effect of light upon the chlorophyll and the plastids in the latter. Ex- 



