INTRODUCTION TO PLANT HISTOLOGY 475 



they form by the junction of numerous cells together, when they are called 

 latex vessels. Alternatively latex tubes are formed by the enormously 

 extended growth of a single cell, first distinguishable in the embr\'o plant, 

 which ramifies through all the tissues, continuing to grow and branch as 

 long as the plant lives. Latex tubes have smooth cellulose walls, but latex 

 vessels have rough walls due to the breakdown of the component cells. 



Latex itself is a complex emulsion, made up of droplets of many sub- 

 stances — resin, tannin, oils, proteins, wax, etc. — dispersed in a watery medium. 

 In the Euphorbiaceae starch grains of a peculiar dumb-bell shape are also 

 included (Fig. 466). Most important, how^ever, is the hydrocarbon substance 



Fig. 466. — Euphorbia splendens. Latex tube showing 

 thickened walls and dumb-bell shaped starch 

 grains. 



called caoutchouc, one of the polyterpenes, which is the mother substance 

 of rubber. It is generally present in latex, but only in a few cases in sufficient 

 amount to be commercially valuable. (See Volume IV, under Rubber in 

 " Economic Botany.") 



Special Cell Contents 



In studying Plant Histology, cells will often be noticed which contain 

 special substances deposited in them, substances which, unlike the reserve 

 foods such as starch, oil and protein, are not directly concerned with the life 

 of the cell. 



Prominent among such special cells are those containing tannin, in 

 brown amorphous masses. These are usually dead cells in old pith or bark 

 tissues, but the tannin so deposited is previously to be found dissolved in the 

 cell-sap of many cells which are still living. 



