108 BOTANY r.uir i 



expanded in all directions, and possess a layer of protoplasm lining the 

 wall, and other cell contents. Typical prosenchymatous tissue, on the 

 other hand, consists of thick-walled, elongated cells, either in the form 

 of fibres or spindle-shaped cells, with interlocking, pointed ends, and 

 with little or no protoplasmic contents. A parenchymatous tissue, in 

 which the cells are thick-walled and elongated, resembles prosenchyma, 

 but may be distinguished from it by the absence of pointed cell 

 terminations, and especially by the greater abundance of protoplasm. 

 Thin-walled prosenchyma is not, on the other hand, necessarily lacking 

 in protoplasm, but is characterised by its pointed and interlocking cells. 

 General Features of Mature Tissues. — The cells in a tissue may 

 either fit closely together (Fig. 77), leaving no openings or spaces, or 

 so-called intercellular spaces may be left between the individual 

 cells. Where cell filaments are interwoven to form a tissue, the inter- 

 cellular spaces are represented by the openings left between the loosely- 

 woven filaments (Fig. 109). In tissues resulting from cell division the 

 intercellular spaces arise subsequently, as the partition wall between 

 two cells formed by cell division is originally a simple membrane. 



Sucli a partition wall may ultimately split and so give rise to intercellular 

 spaces, but tliis only occurs after it has been thickened. The cause of such splitting 

 is to be found in the swelling of the primary wall consisting of pectinose material, 

 and the tendency of the cells to assume a rounded shape. The formation of inter- 

 cellular spaces commences, therefore, at the cell corners. 



The simplest and at the same time most frequent intercellular 

 spaces are triangular or quadrangular in outline, as seen in cross-section 

 (Figs. 72 i, 81 i). In cases where special portions of adjoining cells 

 are in extremely energetic growth, intercellular chambers and passages, 

 of more or less regular shape, may be formed between them. If the 

 growth of adjoining cells is very unequal, it may lead to a complete 

 separation of their cell walls ; or the cells, or even a whole system of 

 tissues, may be stretched and torn apart. It is by such a process that 

 hollow stems are formed. Intercellular spaces arising from a splitting 

 of adjoining cell walls are termed SCHIZOGENIC ; those formed by 

 tearing or dissolution of the cells themselves are called lysigenic 

 INTERCELLULAR spaces. In many cases intercellular spaces that begin 

 as schizogenic are increased in size by destruction of surrounding cells. 

 Most intercellular spaces contain only air, although in special instances 

 they may contain water or excreted products, such as gum, mucilage, 

 resin, or ethereal oils, and in other still rarer cases latex. Schizogenic 

 intercellular spaces are usually filled with air, and form the ventilating 

 system of the plants, while the lysigenic spaces often contain either 

 water or secretion products. 



Of the schizogenic, intcrcelluhir spaces, tliose filled with ethereal oils or resin, on 

 account of their frequency, should be particularly noticed. Short cavities and 

 longer passages, or ducts, containing etliercal oils, are to be found in the stems, 



