54 



ORGANOGRAPHY. 



but are merely prolongations externally of the cellular envelope 

 of the bark. Their use is altogether unknown, although various 

 functions were ascribed to them at ditferent periods before 

 their structure was properly understood. 



6. Intercellular System. — Having now described the dif- 

 ferent varieties of cells, and the modifications which they undergo 

 when combined so as to form the tissues, we have in the next 

 place to allude to certain cavities, &c., which are placed between 

 their sides. These constitute the intercellular system. The cells 

 being, in the greater majority of cases, bounded by rounded 

 surfaces, or more or less irregular outlines, it must necessarily 

 happen that when they come in contact they can only touch 

 at certain points, by which interspaces will be left between 

 them, the size of which will vary, according to the greater or 

 less roundness or irregularity of their surfaces. When such 

 spaces exist as small angular canals running round the edges 

 of the cells and freely communicating with each other, as is espe- 

 cially evident in round or elliptical parenchyma (fig. 1), they 

 are called intercellular passages or canals ; but when they are of 

 large size, as in spongiform tissue, intercellular spaces (figs. 51 

 and 80, c). In most cases these spaces and canals are filled with 

 air, and when they occur in any organ exposed to the atmo- 

 sphere which possesses stomata, they always communicate with 

 them, by which means a free passage is kept up between the atmo- 

 sphere and the air they themselves contain. The laticiferous 

 vessels, as we have already seen, appear in many cases at least 

 to be formed out of the intercellular canals. In water plants these 

 intercellular spaces are commonly of large size, and bounded by 

 a number of small cells regularly arranged, by which they are 

 ])rcvented from communicating with each other, or with the 

 external surfaces of the organs in Avhich they occur (fig. 135). 

 Such are commonly termed air-cells or 

 cavities. In these plants they evidently 

 fulfil the important services of enabling 

 tlicm to float, and supplying their interior 

 with air. In other instances we find large 

 air cavities, as in the stems of Grasses, 

 Eushes, Umbelliferous Plants, &c., which 

 ajjpear to have been formed by the de- 

 struction of the intenial tissue by the 

 more ra])id growth of their outer por- 

 tions. These intercellular canals or 

 sjiaces frequently act as recejitacles for the 

 jteculiar secretions of the plant; in which 

 case they are termed litscrvoirs or liecep' 

 In many cases these arc closely allied to 

 the internal glands already described (figs. l.'iOand 131). They 

 vary much in form, but m*e usually more or less elongated. In 



Fif). l.Vi. Air-ceUs or 

 cavities liom tlic ntimof 

 Limnocfiftiig Vlinnicvi, 



tides of Secretion. 



