SECT. I MORPHOLOGY 109 



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

 in these, and the oil-ducts form the characteristic markings (vittae) on their fruits. 

 The Conifers are esj)ecially characterised by resin-ducts (Fig. 151 A, /(.), which, 

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

 the excretion from tlie 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 

 suednifera). The formation of gum in lysigenic gum cavities commences in the 

 cell contents and then extends to the cell walls ; 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 C^). Latex does not occur in lysigenic intercellular spaces. 



The separating walls resulting from cell division are simple lamellae. 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. 72 m, 76 m). In soft 

 tissues the middle lamella, according to Devaux ("2), is composed for the most 

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

 boiling soft tissues in water, the cells may often be easily isolated, owing to the 

 consequent swelling and dissolution of the middle lamella. In ripe fruit, an 

 isolation of the cells frequently takes place spontaneously, through the dissolution 

 of the middle lamella. A lignified middle lamella, on the other hand, seems able 

 to withstand more effectually the action of oxidising agents. Consequently, it 

 is possible, by subjecting a section of pine -wood to the action of Schulze's 

 macerating mixture (potassium chlorate and nitric acid), and subsequently 

 treating with concentrated sulphuric acid, to remove all secondary and tertiary 

 thickening layers, so that only the middle lamellae remain as a delicate network. 

 If the macerating process be continued fo,r a longer time, without the subsequent 

 treatment with sulphuric acid, the middle lamellae become finally dissolved. The 

 thickening layer will then be left free from all lignification. Schulze's macerating 

 method may accordingly be employed to isolate the elements of lignified tissues. 

 The peculiar relation of the middle lamella towards chemical reagents gave rise 

 at one time to the presumption of a special intercellular substance, which, like a 

 glue, bound together the cells of a vegetable tissue. The supplementary deposition 

 of pectic compounds in the middle lamellae (p. 77) frequently gives rise to the 

 formation of rod -like protuberances and excrescences, which project into the 

 intercellular spaces, or tliese spaces may be iilled up by the formation of gussets 

 (Fig. 76 C. m*). 



