98 BOTANY PART i 



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 de- 

 struction of surrounding cells. Most inter- 

 cellular 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 



FIG. io7.-Transverse section of ra rer cases latex. Schizogenic intercellu- 



the sclerotium of Claviceps , 11 .en j -ii. i 



purpurea. (x 300.) lar 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 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 he 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 (vitte) on their fruits. 

 The Conifers are especially characterised by resin-ducts (Fig. 141 A, /t), which, 

 even during their formation by the separation of the cell walls, seem to lill with 

 the 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 laterally, 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 

 disorganised by the gradual absorption of the cell walls. In this way are formed 

 the receptacles filled with ethereal oils, in the Orange and Lemon. The formation 

 of the so-called resin-galls, in the case of coniferous trees, is preceded by the 

 formation of abnormal tissues, which afterwards 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 gum, as, for example, the gum on 

 Cherry trees. Latex does not occur in lysigenic intercellular spaces. 



The separating walls resulting from cell division are simple lamellaj. In 

 tissues which have arisen by cell division these lamellae are common to the cells 

 they separate. That part of the partition wall between two cells which stands 

 out so distinctly in a cross-section does not. consist of the original primary cell 

 wall alone. It is made up of both the primary wall and the primary thickening 

 layers. The former is called the MIDDLE LAMELLA (Figs. 66 m, 70 m). In soft 

 tissues the middle lamella, according to DEVAUX ( 10 ), is composed for the mo>t 

 part of pectic substances ; in woody and corky tissues it is also lignified. By 



