MORPHOLOGY 83 



The thickening layers of sclerenchynia fibres may be either lignified or un- 

 lignified ; those of tracheids are always lignified. The characteristic thickened 

 walls of the vasiform tracheids serve to sustain the pressure of the surrounding 

 living cells. 



Of all the cells in the more highly organised plants, the latex cells 

 or milk cells, also spoken of as latex tubes, attain the greatest length. 

 In the Euphorbiaceae, Urticaceae, Apocyneae, and Asclepiadaceae they arise 

 from cells which are already differentiated in the embryo. Grow- 

 ing as the embryo grows, they branch with it and penetrate all its 

 members, and may thus ultimately become many metres long. The 

 latex cells themselves have, for the most part, unthickened smooth 

 elastic walls which give a cellulose reaction. They are provided with 

 a peripheral layer of living cytoplasm and numerous nuclei. Their 

 sap is a milky, usually white fluid, which contains gum-resins, i.e. a 

 mixture of gums and resins, caoutchouc, fat and wax in emulsion. 

 In addition, they sometimes hold in solution gums, tannins, often 

 poisonous alkaloids, and salts, especially calcium malate, also, in the 

 case of Ficus Carica and Garica Papaya, peptonising ferments. In the 

 latex cells of the Euphorbiaceae there are also present in the latex 

 peculiar dumb-bell-shaped starch grains. On exposure to the air the 

 milky sap quickly coagulates. In the adjoining figure (Fig. 90, D) 

 is shown a portion of an isolated latex cell dissected out of the stem 

 of an Asclepiadaceous plant, Ceropegia stapelioides. 



Special cells, which differ in form, contents, or in their peculiar 

 wall thickenings from their neighbouring cells, are distinguished as 

 idioblasts. If strongly thickened and lignified, they are called sclerotic 

 cells (stone cells) or sclereids. They generally contain some secreted 

 substance. In a previous figure (Fig. 80) an idioblast, containing a 

 bundle of raphides, is represented. Idioblasts, resembling tracheids 

 and functioning as water reservoirs, are found between the chlorophyll- 

 containing cells in the leaves of some of the Orchidaceae. 



Cell Fusion 



Cell fusion occurs much less frequently in plants than in animals. 

 Yet in all sexually differentiated plants, just as is the case in animals, 

 fertilisation depends for its consummation on the fusion of living pro- 

 toplasts. A fusion occurring between naked cells has already been 

 noticed in describing the formation of a plasmodium by the Myxomy- 

 cetes (Fig. 52). When the hyphae of Fungi touch one another, their 

 walls are often absorbed at the point of contact, and the living contents 

 of two different hyphse become united. In higher plants a similar 

 fusion takes place in latex vessels and in sieve-vessels. Latex vessels 

 have the same structure and contents as latex cells. Their occurrence, 

 like that of latex cells, is limited to a few distinct plant families, 



