80 BOTANY part i 



tracheides with a wide lumen and thin walls, serving, like typical 

 tracheides, as water-carriers, are distinguished as vasiform or vascular 

 TRACHEIDES (Fig. 85 C). They are characterised by the annular, 

 spiral, or reticulate markings of their thickening layers, and may 

 also be provided with bordered pits. The walls of tracheides are 

 always lignified, while those of the sclerenchyma fibres may or may 

 not have undergone this change. 



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, Moraceae, Apocynaceae, and Asclepi- 

 adaceae they arise from cells which are already differentiated in the 

 embryo. Growing 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 gnm-resins, i.e. mixtures of gums and resins, 

 caoutchouc, fat and wax in emulsion. In addition, they sometimes 

 hold in solution enzymes, leptomin, tannins, often poisonous alkaloids, 

 and salts, especially calcium malate, also in the case of Ficvs Carica 

 and Carica Papaya peptonising ferments. Proteid granules often 

 occur in the latex, and in the latex cells of the Euphorbiaceae there 

 are also present peculiar dumb-bell-shaped starch grains. On ex- 

 posure to the air the milky sap quickly coagulates. In the adjoining 

 figure (Fig. 85 D) is shoAvn a portion of an isolated latex cell dissected 

 out of the stem of an Asclepiadaceous plant, Ceropegia stapelioides. 



Special cells which diff"er 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 sclereides (Fig. 168). They often 

 contain ferments ; in the Cruciferae and some other orders myrosin is 

 thus present, while Primus laurocerasus contains emulsin. For the most 

 part they contain excreted substances such as tannins and calcium 

 oxalate. In Fig. 71 an idioblast, containing a bundle of raphides, is 

 represented. Idioblasts, resembling tracheides and functioning "as 

 water reservoirs, are found between the chlorophyll-containing cells 

 in the leaves of some of the Orchidaceae and Cactaceae. 



Size of Cells. — The corresponding cells of equivalent members 

 of the same plant are usually of nearly the same size, even when the 

 members show a variation in size. In meristematic cells there is a 

 definite relation between the amount of cytoplasm and the size of the 

 nucleus. This has been recognised as the nucleus-plasm relation by 

 R. Hertwig (82). 



DifTepentiation of the Protoplasts. — In organisms composed of 

 one or of few cells the sej^arate parts of the same protoplasts 



