BOTANY 



PART I 



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

 section (Figs. 81 i, 87 i). In cases where special portions of adjoin- 

 ing cells are in extremely energetic growth, intercellular chambers and 

 passages, of more or less irregular 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 accord- 

 ingly termed schizogenic ; those formed by 

 tearing or dissolution of the cells themselves 

 are called lysigenic intercellular spaces. 

 Most intercellular spaces contain only air, 

 although in special instances they may con- 

 tain water or excreted products, such as 

 gum, mucilage, resin, or ethereal oils, and in 

 other less frequent cases latex. Schizogenic 

 intercellular spaces are usually filled with 

 air, while the lysigenic spaces contain almost 

 always either water or excretion products. 



Fig. 99. — Longitudinal section 

 of the cortical cells of Nerium 

 Oleander, after treatment with 

 chloroiodide of zinc and me- 

 thylene blue to show the cyto- 

 plasmic connection between 

 the cells. (After Kienitz- 

 Gerloff, x 900.) 



Of the schizogenic intercellular spaces, those filled 

 with ethereal oils or resin, on account of their 

 frequency, should be particularly noticed. Short 

 cavities and longer passages, or ducts, containing 

 ethereal oils, are to be found in the stems, roots, 

 and leaves of numerous plant families. The Umbelliferae are especially rich in 

 these, and the oil-ducts form the characteristic markings (vittse) on their 

 fruits. The Conifers are especially characterised by resin-ducts (Fig. 139, h), 

 which, even during their formation by the separation of the cell walls, seem to fill 

 with an excretion from the cells. The enlargement of such intercellular spaces 

 is accompanied by a division of the surrounding cells, the number of which is thus 

 correspondingly increased. The cells themselves remain thin- walled, and in close 

 contact, but bulge out somewhat into the ducts. Lysigenic intercellular spaces, 

 acting as receptacles for secretions, have the appearance of irregular cavities in the 

 tissue. "Where they contain oil or resin, they develop from a group of cells in which 

 these substances appear in the form of drops. The cell group then becomes dis- 

 organised by the gradual absorption of the cell walls, beginning with those of the 

 cells in the centre of the group. In this way are formed the receptacles filled with 

 ethereal oils, as, for example, those in Didamnus (Rutaceae), (Fig. 116), and in 

 Awrantieae, as in the Orange and Lemon. The exudation of resin, in the case 

 of coniferous trees, is preceded by the formation of abnormal tissues, which after- 

 wards become converted into resin. Such was also the origin of amber, which is 

 the fossil resin of the Amber-fir (Picea succinifera). The formation of gum in 

 lysigenic gum cavities is due to the modification of the cell walls, and either 

 normal tissues participate in this process, as in the case of the gum-arabic of 

 the Acacia, or abnormal tissues are first developed and then transformed into 



