SECT, ii PHYSIOLOGY 221 



The Mechanism of Gaseous Exchange 



The gaseous exchange between one of the lower plants (or a submerged Phan- 

 erogam) and the surrounding medium is carried on by diffusion through the whole 

 surface of the plant. In a more highly organised plant the exchange takes place 

 by means of the stomata. The escape of aqueous vapour and the entrance and 

 escape of carbonic acid gas and oxygen in the processes of assimilation and respira- 

 tion (p. 244) alike take place almost entirely through these openings. But in 

 spite of their enormous number, the total area of the stomatal apertures is only 

 about 1 per cent or little more of the whole surface area, for each individual 

 opening is very small (about O'OOOl sq. mm. in Helianthus). Bnowx and 

 ESCOMBE have, however, shown that the minuteness of the openings, combined 

 with the peculiar distribution of the stomata, results in a greatly accelerated rate 

 of diffusion. Diffusion through a number of fine apertures is much greater than 

 through a single aperture of the same total area. When the small openings are 

 placed about ten times their diameter apart the diffusion is nearly as rapid as when 

 no separating wall is present. The distribution of the stomata in the epidermis 

 very nearly meets these requirements. For example, a square metre of the surface 

 of a Catalpa leaf absorbs about two-thirds the amount of carbonic acid gas taken 

 up in an equal time by the same area of potash solution freely exposed to the air ( 41 ). 



The Utilisation of the Products of Assimilation 



The Formation of Albuminous Substances. The chlorophyll 

 bodies supply plants with organic nourishment in the form of a 

 carbohydrate. Although the greater part of the organic plant sub- 

 stance consists only of carbohydrates, as, for example, the whole 

 framework of cell walls, yet the living, and consequently the most 

 important component of the plant-body, the protoplasm, is composed 

 of albuminous substances. These albuminous substances have a com- 

 position altogether different from that of the carbohydrates. In 

 addition to carbon, oxygen, and hydrogen, they also contain nitrogen, 

 sulphur, and phosphorus, the nitrogen indeed in considerable pro- 

 portion (about 15-19 per cent). THERE TAKES PLACE ACCORDINGLY 



WITHIN PLANTS A NEW FORMATION OF ALBUMINOUS SUBSTANCES. 



There are certain indications that this formation is, in part, accom- 

 plished within the green cells of the leaves, but it must also be carried 

 on in tissues devoid of chlorophyll. 



As little is known concerning the process of the synthesis of the albuminous 

 substances of plants as concerning the formation of the carbohydrates from the 

 carbonic acid and water. It has generally been supposed that they are formed 

 from the carbohydrates and mineral substances already mentioned, since these are 

 known to be transported to the region where the formation of protoplasm occurs, 

 and are there consumed. The carbohydrates utilised in this process seem to be 

 principally GLUCOSE (both grape-sugar, dextrose, C 6 Hi 2 6 + H 2 0, and fruit-sugar. 

 Isevulose, C B H 12 O 6 ) and MALTOSE (C^H^On x H 2 0) together with benzole derivatives ; 

 whatever may be the form of the original carbohydrate, whether starch, inulin, 

 cane-sugar, reserve-cellulose, or glycogen, glucose or maltose is always first formed 

 from it. 



