MORPHOLOGY 



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 lamellae. 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, and is called 

 the middle lamella (Figs. 81 m, 83 m). In soft tissues the middle lamella, 

 according to Maxgin, is composed of pectose combined with calcium (calcium 

 pectate) ; in woody and corky tissues it has the same composition, but is also 

 lignified. By boiling soft tissues in water, the cells may often be easily isolated 

 through 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. Conse- 

 quently, it is possible, by subjecting a section of pine -wood to the action of 

 Schulze's macerating mixttjke (potassium chlorate and nitric acid), and subse- 

 quently 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 for a longer time, without the 

 subsequent treatment with sulphuric acid, the middle lamellae become finally dis- 

 solved. The thickening layer will then be left free from all lignification, and will 

 in that condition give the blue cellulose reaction with chloroiodide of zinc (p. 80). 

 Schulze's macerating method may accordingly be employed to isolate the elements 

 of lignified tissues. The inexplicable attitude of the middle lamella towards 

 chemical reagents gave rise at one time to the presumption of a peculiar inter- 

 cellular substance which, like a glue, bound together the cells of a vegetable 

 tissue. The supplementary deposition of pectose in the middle lamellae (p. 80) 

 frequently gives rise to the formation of rod -like protuberances and excrescences, 

 which project into the intercellular spaces, or these spaces may be filled up by the 

 formation of gussets (Fig. 83, 0, m*). The yellowish brown colour assumed by the 

 pectose deposited on the walls of intercellular spaces, on treatment with chloroiodide 

 of zinc, led to the erroneous supposition that the intercellular spaces in plants were 

 lined by a thin layer of living cytoplasm. 



Vegetable tissues may be divided into two groups, paeenchyma 

 and PROSENCHYMA, between which, however, no sharp distinction 

 can be made. A typically developed parenchymatous tissue is one in 

 which the thin-walled cells are equally expanded in all directions, and 

 are, for the most part, rich in protoplasm. 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 pros- 

 enchyma, 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. 



An undifferentiated tissue, the cells of which are still capable of 

 division, is termed embryonic tissue, or meeistem. The meristem of 

 embryonic rudiments and of the growing point is called pkomeristem, 



