292 BOARD OF AGRICULTURE. 



consists of a colorless membrane of cellulose. This is lined in 

 the young and living cell by a membrane like that within the 

 shell of an egg, which is called the primordial utricle, or the 

 original sac. "Within this is inclosed the nitrogenous vital fluid 

 or protoplasm, corresponding to the white of an egg, in whicli 

 floats the nucleus, a minute globular body, analogous to the 

 yolk. Vegetable growth results from the multiplication and 

 enlargement of these cells ; and the macrocystis pysifera, a sea- 

 weed, which in the Antarctic Ocean sometimes attains a length 

 of seven hundred feet, may be regarded as merely an aggrega- 

 tion of simple cells, and limited in size only by the capacity of 

 its stem to resist the force of the waves. In the higlier orders 

 of plants these spherical cells are variously modified and 

 arranged to accomplish certain important objects. Thus to 

 form the outer bark, as in the canoe birch, they are compressed 

 and flattened ; to build up the stout woody fibre, as in the 

 hickory, they are elongated, and the tapering extremities lapped 

 upon each other ; to constitute the ducts for tiie circulation of 

 air through the plant they are joined into continuous tubes; 

 while solidity is imparted to the heart wood of trees and the 

 stones of fruits by the thickening of the cell walls. 



The multiplication of cells arises in some cases from the 

 development of new cells within the fluid contents of unicellu- 

 lar plants, which are thereby destroyed, but at the same time 

 reproduced and multiplied. Ordinary plant-growth, however, 

 results from the spontaneous subdivision of living cells on or 

 near the surface, and the subsequent enlargement of their sub- 

 divisions to the full size of the original mature cell. This proc- 

 ess of vegetation, by which living plants continue to increase 

 in bulk, is, therefore, simply the reproduction of cells by cells. 

 In this way the roots which supply food, the branches which 

 serve to expand the foliage to the vitalizing influences of the 

 sun and the atmosphere, and the leaves, which are the organs 

 of digestion and respiration, are perfectly developed in the 

 relative and absolute proportions of the species to which the 

 individual may belong. This miracle of growth, where all the 

 organs are present and all the circumstances favorable, is utterly 

 beyond our comprehension and must command the admiration 

 of every contemplative person. But the facts relating to the 

 reproduction of plants and the preservation of species and 



