LATEX 



155 



more or less coinciding with that of a cross-connection between 

 the vertical components of the system ; moreover, owing to the 

 thin walls of the laticiferous vessels, the pressure of the surround- 

 ing elements leads to distortion. The dense irregular network 

 resulting from the numerous cross-connections is a very prominent 

 feature in a radial longitudinal section (Fig. 78, C, /.). 



Latex, like milk, is an emulsion, the fluid basis of which is 

 a solution of diverse 

 substances (mineral salts, 

 sugars, proteins, tannins, 

 etc.). In some cases it 

 includes an important 

 active principle of the 

 plant ; for example, in 

 the Opium Poppy (Pa- 

 paver somniferum) the 

 alkaloid morphine. The 

 suspended particles in- 

 clude on the one hand 

 oil-drops, on the other 

 granules of resin, gum, 

 protein, and caoutchouc, 

 whilst, as already noted, 

 starch-grains occur in 

 the latex of the Spurges. 

 On exposure to air latex 

 as a general rule congeals 

 rapidly, a change often 

 accompanied by dis- 

 colouration 1 ; the "set- 

 ting " is partly due to evaporation of water, but mainly to 

 a confluence of the oil-globules and suspended particles. The 

 coagulation of the latex, like that of blood, is of advantage in 

 protecting and rapidly covering a wounded surface ; moreover, 

 the " dressing" in this case is even antiseptic. The laticiferous 

 elements further serve as food-reservoirs, and in this connection 



1 This feature is especially marked in the case of the latex of the 

 Lacquer- tree (Rhus vermicifera), where the action is due to an oxidising 

 enzyme (cf . p. 55) which converts the white juice into a dark shining varnish. 



B 



FIG. 78. Laticiferous vessels in the root 

 of the Salsify (Tragopogon) . A, Dia- 

 grammatic representation of a small 

 part of a transverse section, showing 

 the distribution of the laticiferous 

 vessels (/.), in relation to the cam- 

 bium (c.) t and the vessels of the 

 xylem (V .). B, a small part of the 

 secondary phloem enlarged. C, Longi- 

 tudinal section, p., parenchyma. 



