34 THK ELEMENTARY STRUCTURE OF PLANTS. 



as well as in living ones. Tae law is, that when two fluids of un- 

 equal density are separated by an organic membrane, or by any thin 

 and porous partition, an interchange takes place, — more or less 

 rapidly according to the thinness of the intervening partition and 

 the difference in the density of the fluids on the two sides, — a small 

 quantity of the denser fluid passing into the lighter, but a much larger 

 portion of the lighter passing into the denser ; and this continues 

 until the two fluids are brought to the same density. Hence, as 

 the cells of a living plant always contain organizable or assimilated 

 matter (mucilage, protoplasm, &c), which especially abounds in 

 young and growing parts, the cells of the rootlets are always able 

 to imbibe the ordinary moisture which is presented to them in the 

 soil ; and by diminishing the portion of water, or in any other way 

 increasing the density of the liquid contents of the cells of any part 

 of the plant, a flow may be attracted into them. 



41. Increase of Cell-walls in Thickness. Up to a certain point, the 



walls of cells thicken as they grow by the incorporation of new 

 matter interstitially into their substance. After attaining, for the 

 most part rapidly, a definite size, the cell ceases to enlarge, and its 

 wall no longer incorporates new materials. Some cells remain with 

 exceedingly thin and delicate walls. But in most cells that make 

 part of the permanent structure of a plant, the cell-membrane con- 

 tinues to thicken long after it has ceased to enlarge. Then the 

 new matter can no longer be incorporated with the old ; but 

 the thickening is now effected by its deposition on the inner sur- 

 face of the original membrane, between it and the protoplasmic 



slightly sweet ; showing that a small quantity of sirup has passed through the 

 pores of the membrane into the water without, while a much larger portion of 

 water has entered the tube. The water will continue to enter the tube, and a 

 small portion of sirup to leave it, until the solution is reduced to the same 

 strength as the liquid without. If a solution of gum, salt, or any other sub- 

 stance, be employed instead of sugar, the same result will take place. If the 

 same solution be employed both in the vessel and the tube, no transference or 

 change will be observed. But if either be stronger than the other, a circulation 

 will be established, and the stranger solution will increase in quantity until the 

 two attain the same density. If two different solutions be employed, as, for 

 instance, sugar or gum within the tube, and potash or soda without, a circula- 

 tion will in like manner take place, the preponderance being towards the denser 

 fluid, and in a degree proportionate to the difference in density. Instead of ani- 

 mal membrane, any vegetable matter with fine pores, such as a thin piece of wood, 

 or even a porous mineral substance, may be substituted, with the same result. 



