ASSIMILATION IN SHADE 151 



One particular factor is always limiting, but that factor may 

 change a dozen times with the varying conditions of an 

 English summer day. 



Shade Plants. — These experiments all deal with plants 

 which receive at least intermittent sunlight. It would be a 

 matter of great interest to know how plants living in constant 

 shade behave. A good deal of attention has been devoted 

 to this question, but, unfortunately, even in the most recent 

 work, that of Lundegardh (1921, 1922), the experimental 

 methods are not completely adequate ; in particular, the 

 importance of establishing the fundamental relation to 

 temperature has not been realised. Lundegardh finds that 

 at normal carbon dioxide content the supply of the gas 

 limits assimilation in a shade plant, Oxalis, when the 

 illumination reaches a value equal to one-tenth sunlight, 

 while for a sun plant. Nasturtium, the rate increases up to 

 one-quarter sunlight. He attributes the difference to lesser 

 efficiency in the diffusion of carbon dioxide into the chloro- 

 plasts of the shade plants. The chloroplasts of Nastur- 

 tium, for example, are more numerous and smaller and the 

 cells are much larger than those of Oxalis. There is a 

 much greater area of cell surface (through which carbon 

 dioxide diffuses) per unit volume or unit area of chloroplast 

 in the Nasturtium leaf, and the area of chloroplast surface 

 is much greater in relation to the chloroplast volume. 

 Further, the Nasturtium leaf has more numerous stomata, 

 and though these are smaller, the diffusive capacity of the 

 Nasturtium leaf must be at least 2I times that of the 

 Oxalis leaf. With equal supply of carbon dioxide in 

 the atmosphere, the supply to the assimilating surface will 

 be superior in Nasturtium, which will therefore be able to 

 make use of a higher intensity of light. 



Lundegardh and Boysen- Jensen (19 18) both find that 

 in thin-leaved shade plants, e.g. Oxalis and Lychnis dioica, 

 the respiration is much less than in thicker-leaved sun plants, 

 e.g. Atriplex and Sinapis. The light intensity at which as- 

 similation, with normal carbon dioxide supply, just balances 

 respiration is always much lower in the shade plant. 



