20 CELLULAR TISSUE. 



tion, the cohesive resistance of the valves is overcome and a 

 sudden bursting of the capsules is the consequence ; the equi- 

 librium of the pressure being produced by the coiling up of 

 the valves. The opening and shutting of flowers at certain 

 hours of the day is undoubtedly another effect of the same 

 cause. If we examine a plant, which opens and shuts during 

 the day, we may readily observe a contraction of the cellular 

 tissue on one side of the flexure, and a turgessence of the 

 cells on the other side. 



17. The cellular tissue is endowed with the power of re- 

 producing itself. This is abundantly proved by the existence 

 of vegetables consisting entirely of these cells, and the extreme 

 rapidity with which they are sometimes generated, is striking- 

 ly illustrated by an example given by Prof. Lindley, of a 

 mushroom, the cells of which he estimated to be produced at 

 the rate of four billions per hour. 



18. This tissue, at first soft and mucilaginous, becomes, by 

 age, of a very different consistence, varying remarkably in its 

 composition in different vegetables, and in different parts of 

 the same vegetable. It always commences its existence, as 

 we before remarked, possessed of the same organization, but 

 in its maturity it may become the white, thin, transparent 

 vessicle of the pith of the elder, or the hardened, thickened, 

 unyielding prosench^'ma of the wood and the liber. These 

 changes are produced by several circumstances, in the elder 

 all the substance of the cell except the exterior vessicle 

 becomes the food of the plant. The consistence of a cell 

 may be increased in several ways. First, by the generation 

 of new cells internally, which of course would increase the 

 density as the number is multiplied. Second, by the diminu- 

 tion of the aqueous part, and consequently proportional aug- 

 mentation of the carbonaceous part. Third, by the combi- 

 nation of the organic substance with an inorganic, earthy, 

 or metallic base which solidifies and transforms the almost 

 mucilaginous incipient cellular tissue, into the firm, elastic, 

 resisting prosenchyma. Fourth, by the deposition of resins, 

 which have little or no affinity for water, and of course in- 

 crease the hardness of the tissue in which they are deposited. 

 We see this hardening of the cellular tissue strikingly il- 

 lustrated in the hard gritty cells of the Quince and Pear. 

 These gritty grains are occasioned by depositions within the 

 cells, and are of the same nature as those that constitute the 

 stone of the Cherry and Peach. 



19. The parenchymous tissue is in general the depository 

 of all the materials Avhich in vegetables administer to the sus- 



