SECT. xiv. SUBS, SPINES, AND HAIRS. 411 



leaf with the descending portion, which is beneath and 

 in contact with it throughout its ramifications. This 

 descending portion at the base of the leaf-stalk, or 

 petiole, becomes continuous with the bundles of the 

 liber. In the upper part of the nerves of the leaf there 

 are spotted vascular ducts, in the lower part there are 

 laticiferous vessels. Those on the upper side carry the 

 rising sap to the green matter, where it is elaborated 

 and matured, and then it passes into the vessels on the 

 under-side of the nerves or veins, which cany it down 

 the liber. 



Buds are generally formed of scales closely imbricated 

 round the young leaves, which are variously folded and 

 firmly packed ; they contain the rudiments of the whole 

 plant, and as in a large tree they are renewed every 

 year, the sources of life are all but infinite. 



The spines with which many plants are armed are of 

 two kinds ; one is permanent, being an excrescence from 

 the wood, as in the blackthorn ; the other proceeds 

 from the bark, and may be stripped off, as in the rose ; 

 both contain silex, and are covered by the skin common 

 to the whole plant. 



Few plants of any kind are without hairs, which are 

 chiefly found on the young shoots, and on the under 

 surface of the leaves. They are either formed of a trans- 

 parent elongated hollow cell, or consist of a number 

 of transparent colourless superimposed cells, sometimes 

 jointed, but more frequently rectilinear. When they 

 sting, as in the nettle, they are set upon a kind of 

 bulb composed of cells which secrete the acrid colour- 

 less liquid which causes the irritation, and when slightly 

 pressed send it through the hair, the point of which 

 breaks off as it enters the skin of the hand. 



The hairs are so transparent that the gyration of the 

 azotized liquid, called protoplasm they contain, has been 

 distinctly traced, a motion so universal in some part 



