Si ORIGIN OP STEM AND WOOD. 



have perished. In brief, the stem is a branch produced by the 

 first leaf-bud which the embryo plant possesses. 



"When the stem is first called into existencej it is merely a 

 vegetable cell, afterwards increased into a small portion of 

 cellular tissue: an organic substance, possessing neither 

 strength nor tenacity, and altogether unsuited to the purposes 

 for which the stem is destined. If the stem consisted exclu- 

 sively of such matter it would have neither toughness nor 

 strength, but would be brittle like a mushroom, or like those 

 parts of plants of which cellular tissue is the exclusive com- 

 ponent, such for example as the club-shaped spadix of an 

 Arum, or the soft prickles of a young Eose branch. Nature, 

 however, from the first moment that the rudiment of a leaf 

 appears upon the growing point of a stem, occupies herself 

 with the formation of woody matter, consisting of tough tubes 

 of extreme fineness, which take their rise near the leaves, and 

 which, thence passing downwards thrpugh the cellular tissue, 

 are incorporated with the latter, to which they give the neces- 

 sary degree of strength and flexibility. In trees and shrubs, 

 they combine intimately with each other, and so form what is 

 properly called the wood and inner bark ; in herbaceous and 

 annual plants, they constitute a lax fibrous matter. No woody 

 matter appears till the first leaf, or the seed-leaves, have begun 

 to act ; it always arises from near their bases ; it is abundant, 

 or the contrary, in proportion to the strength, number, and 

 development of the leaves ; and in their absence is absent also 

 as a general rule. 



The exceptional oases are those of "leafless" plants; that is to say, 

 of plants in which leaves never advance heyond the condition of scales, 

 and usually drop off soon after their formation. To this class belong 

 green succulent plants like StapeUaa, Cacti, and many Euphorbias. 

 Here the bark is excessively developed, and has the colour, texture, and 

 structure ^of leaves, of which it performs the functions. Such plants 

 form true wood, but of little solidity and in small quantity compared 

 with their bulk. It is also found that in them the wood has a lateral 

 communication with every leaf-bud, as that of ordiaary plants has with 

 every leaf. 



When woody matter is first plunged into the cellular tissue 

 of the nascent stem, it forms a circle a little within the circum- 



