GENERAL 181 



oxygen is frequently not directly attainable by the roots,, and similarly 

 the roots of plants grown in water-cultures must obtain a portion of their 

 oxygen from the sub-aerial organs, while the roots of .'-cedlings of Vicia 

 faba, Pisum, &c. 5 appear able when necessary to obtain a sufficient supply 

 of oxygen solely through these channels x . 



The diffusion of air is not prevented by the thin transverse diaphragms 

 which cross the air canals of some aquatic plants, although these cause the 

 transference to become somewhat slower. Such diaphragms give rigidity 

 to the plant, and prevent the entrance of mud, &c., when the older parts 

 die and rot away, or when the rhizomes are accidentally wounded. 



In spite of its importance in gaseous exchange, the development of the 

 aeriferous system is not to be regarded solely from this standpoint, for 

 intercellular spaces, however formed, usually become filled with air, just as 

 dead cells do. Apparently the large central air-spaces present in the 

 stems of grasses, umbellifers, &c., are not produced specially for purposes 

 of gaseous exchange, but rather for economy of construction, for a hollow 

 cylinder with binding pieces at intervals (the nodes) affords a maximum 

 of strength and rigidity with a minimum of constructive material. In 

 aquatic plants again, air-spaces are of importance as floats to support the 

 plant and keep it erect. 



It is therefore hardly to be expected that the degree of development of 

 the aeriferous system in a tissue should precisely correspond to its gaseous 

 requirements, although intercellular spaces filled with air always appear in 

 the tissues at an early stage in the development of the primary meristem. 

 The younger portions of the apical meristem obtain all the oxygen and 

 other substances which they require by diosmotic transference through 

 several layers of cells, and in these and similar extremely active tissues very 

 narrow intercellular spaces suffice to satisfy the demands for oxygen made 

 by extremely energetic respiration. Bearing in mind that traces only of car- 

 bonic acid are present in the air, it is easy to understand why the aeriferous 

 system should in general be well developed in chlorophyllous tissues. 



It is therefore distinctly advantageous not only that in strongly 

 illuminated leaves the aeriferous system should become more marked 

 than in those exposed to feeble light, but also that in amphibious plants 

 the intercellular system should frequently attain its most marked develop- 

 ment when the plants are completely submerged 2 . Nevertheless, the results 

 produced by the external conditions are not in this or any other case to 

 be regarded solely from a teleological standpoint ; and intercellular spaces 

 may serve other purposes, as, for example, when they form secretory 



1 See Pfeffer, Druck u. Arbeitsleistung, 1893, p. 245. 



2 Stahl, Uber d. Einfluss d. sonnigen u. schattigen Standorts auf d. Ausbildung d. Laubblatter, 

 1883, p. 17 ; Schenk, Biol. d. Wassergewach.se, 1886, and Jahrb. f. wiss. Bot., 1889, Bd. xx, p. 526 ; 

 Goebel, Pflanzenbiol. Schilderungen, 1893, Bd. II, p. 255. and the literature here quoted. 



