LATICIFEROUS VESSELS. 



173 



dissolved and undergoing further chemical changes, they play a most important part 

 in the metabolism of the plant. As however the occurrence of laticiferous vessels 

 themselves is very variable, even within narrow circles of alliance ; so also the 

 chemical composition appears to be variable in the highest degree, from species 

 to species and from genus to genus even within the families concerned. The 

 substances mentioned as contained in the latex may be present or wanting ; or one 

 or another of them may predominate 

 or be reduced to a minimum. 



The laticiferous vessels themselves 

 are always so narrow that they can never 

 be seen on a transverse section of the 

 organ with the unaided eye. The micro- 

 scope, however, shows that they may be 

 of very different diameter in the same 

 plant. In the roots, shoot-axes, and 

 nerves of the leaves, run thicker tubes, 

 from which thinner and yet thinner ones 

 arise. The substance of the walls of the 

 tubes always consists of soft cellulose, 

 sometimes capable of swelling : they are 

 never lignified, suberised, or otherwise 

 essentially altered by infiltration. One 

 of the most prominent characteristics of 

 the laticiferous vessels is their continuity 

 throughout the whole plant, or at any 

 rate over wide areas. This may obvi- 

 ously, even if not in every point, be 

 closely compared with the vascular sys- 

 tem of an animal. This continuity is 

 also the reason why a relatively con- 

 siderable quantity of sap flows from a 

 small injury at any part of the plant, in 

 spite of the narrow diameter of the 

 laticiferous vessels ; for their walls are 

 under high pressure on all sides, which 

 is brought about by the turgescence 

 of the surrounding tissues. On the 

 injury of any tube, therefore, the sap 

 is pressed forward even from distant 

 parts towards the opening. 



According to their origin and form, two kinds of laticiferous vessels are to 

 be distinguished — the segmented and the unsegmented. 



The segmented laticiferous vessels, which occur in the Cichoriacea; (par- 

 ticularly fine in the cortex of Scorzonera), in the Papaveracea?, Campanulaco:v, 

 etc., originate in the embryo, and in the embryonal tissue of the growing 

 point, in the following manner. In certain scries of cells which arc previously 



176.- 



4 Tangential longitudinal section through the root 

 of Scoyzo)tcra hisfanica. In the parencliymatous tissues run 

 numerous laticiferous vessels, anastomosing laterally. K small por- 

 tion of a laticiferous vessel with adjacent parenchyma, more highly 



magnified. 



