ELEMENTARY STRUCT CRE. 



37 



or passages, lying in no definite position with regard to the 

 other tissues (figs. 73 and 74), and anastomosing or uniting 



Figs. 73 and 74. 



freely with each other like the veins 

 of animals, from which peculiarity 

 they may he at once distinguished 

 from other vessels. In the pith of 

 the Elder, according to Henfrey, 

 unbranched laticiferous vessels oc- 

 cur. When first formed, their 

 sides are very thin and the ves- 

 sels are also exceedingly minute, 

 but they become large and thick- 

 sided as they increase in age, but 

 rarely present any pits or spiral 

 deposits in their interior, as is the 

 case in the thickened cells and ves- 

 sels already described. A common 

 size is the -^ho ^^ ^^ ^^^^^ ^^ ^^" 

 ameter. They derive their name 

 from containing a watery fluid called 

 latex, which Avhen exposed to the air becomes milky, and is either 

 white, as in the Dandehon, Spurges, Poppy, India-Rubber, 

 Lettuce, &c.; or coloured, thus yellow latex is well seen in the 

 Celandine. The latex has a number of granules or globules 

 floating in it, which are composed of caoutchouc, or analogous 

 gum-resinous matters, and occasionally, mixed with them may 

 be observ^ed peculiar-shaped starch granules, as in Euphor- 

 bia {fig. 75). Laticiferous vessels were first discovered by 



Fig, 75. 



Fig. 75. Latex 

 vessels from 

 a species of 

 Euphorbia; 

 the latex 

 containing 

 starch grains 

 of a peculiar 

 form. From 



Schultz, who also described the latex 



as constantly circulating in them, and 



hence the term Cinenchyma or moving 



tissue has been apphed to them. The 



movements thus described by Schultz 



will be aUuded to hereafter, when 



speaking of the physiology of cells. 



These vessels occur especially in the 



inner bark of Dicotyledons, in the pith, iieufrey. 



and in the stalks and veins of leaves. 



They are also to be found in the vascular bundles of ^lonocoty- 



ledons and all parts which are prolonged from them. In Acoty- 



ledons they exist only in the higher orders. 



Their nature or origin is by no means well ascertained. By 

 some vegetable anatomists they are considered to be formed, like 

 the ordinary pitted vessels, from rows of cells arranged in 

 various directions with respect to each other, the partitions 

 between their cavities being more or less absorbed, so that 

 they communicate freely together. Others again, as Mohl 

 and Henfrey, consider them as passages between the cells, 

 d3 



