ISOLATION OF CELLS. 75 



termed intercellular spaces, or lacunae. In the ordinary succulent tissues of Stems, 

 Roots, Fruits, etc., these intercellular spaces are relatively very small, since the cells 

 of these portions of the plant, at first fitting closely on all sides, separate a little from 

 one another only here and there. It often happens, however, especially in aquatics 

 and bog-plants, that the large cells of the succulent tissue (parenchyma) become 

 separated from one another in layers, so that large chamber-like cavities arise, which 

 are separated from one another by simple layers of cells. An excellent example of 

 this, easily perceptible even with the unaided eye, is presented by the leaf-sheaths 

 of the species of Musa. 



This mutual isolation of cells, resulting spontaneously during growth, may 

 under certain circumstances be produced artificially, especially when the partition- 

 walls are thick ; either simply by means of continued boiling in water, or by long 

 maceration in a mixture of chlorate of potash and nitric acid, or even by slow 

 rotting. In the excrement of plant-eating animals, also, we find the undigested cells 

 often completely isolated from one another under the influence of the gastric and 



...(-'— rtj 



Fig. 72. — Longitudinal section tlirougli the growing point of a winter bud of Abies pectniata (x about joo). 

 5 the apex of the growing point ; b h youngest leaves ; r cortex ; m pith. 



intestinal juices. In such cases, a splitting of the originally simple partition wall 

 of neighbouring cells is effected by external influences; or a thin layer, the so- 

 called middle lamella, lying in the thick partition wall, is dissolved. However, 

 even in the normal course of vegetation, and especially in the formation of repro- 

 ductive organs, a complete isolation of the cells occurs in such a manner that, 

 finally, they become emptied out of the reproductive organs in the form of a more 

 or less fine dust. The pollen from the opened anthers of the stamens of flowering 

 plants, for instance, consists of such isolated cells ; and occasionally, as in the 

 Pines and many Nettle-like plants, these are carried away in great quantities by 

 the wind. The spores of most Cryptogams are further examples. The fine heavy 

 dust, bought in the apothecaries' shops under the name of Lycopodium, consists 

 entirely and simply of such spores ; and similarly we can shake out of any moss 

 capsule, or blow away from a tuft of the commonest mould {Penicillium glaticum), 

 a still finer powder. If the unfolded pileus of a ripe Mushroom or Toad-stool is 

 laid with the lower side on paper, it is found, after some hours, that a picture of 

 its lamellated or tubular underside has been formed, — a figure in dust, which has 



