GASEOUS EXCHANGE OF SUCCULENTS 113 



this source sometimes results, especially in algae, in a 

 deposition of calcium carbonate in the cell walls. 



§ 8. Gaseous Exchange of Succulents 



Another group of plants peculiar in their relation to 

 carbon dioxide supply are the succulents. They charac- 

 teristically inhabit dry situations where economy in water is 

 important ; their gas exchange with the external atmosphere 

 is often Hmited. When they respire at night the production 

 of carbon dioxide is small, organic acids — malic acid in 

 the Cactaceas, isomalic acid in the Crassulaceas, and oxalic 

 acid in the Mesembryanthemaceae — being produced instead. 

 This is an incomplete form of respiration with reduced 

 energy production, but it also means the retention of carbon 

 compounds in the plant. During the day these plants 

 carry on assimilation partly at the expense of the stored 

 acids, and only partly at the expense of atmospheric carbon 

 dioxide. During the day the amount of acid in the sap may 

 be reduced to one-tenth of its night value ; and the necessity 

 of gas exchange with the atmosphere is reduced. One conse- 

 quence of this is that the ratio of carbon dioxide absorbed 

 to oxygen given off in assimilation falls considerably below 

 unity, which is the normal value. The metabolism of these 

 succulents is peculiar in other respects, as we shall see later. 



§ 9. Energy Relations of Assimilation 



The leaf is a green light screen or filter which absorbs 

 most of the light falling on it, reflecting or transmitting 

 smaller fractions. The proportion of light absorbed by 

 different types of leaf must be very different ; the thin 

 translucent leaf of a Tropseolum obviously absorbs less than 

 the thick leathery leaves of holly or cherry laurel. Exact 

 measurements have not been made for many plants. 

 Browne and Escombe (i905«) found that the percentage of 

 direct sunlight absorbed ranged from 647 in Polygonum 

 Weyrichii, to 787 in Acer Negundo. The sunflower leaf 



I 



