736 PHYSIOLOGY. 



By the ordinary method of cell-division, cells are in many 

 instances produced with almost inconceivable rapidity. Thus, 

 it has been stated that a fungus of the Tuff-ball tribe has 

 been known to grow in a single night, in damp warm weather, 

 from the size of a mere point to that of a large gourd ; and it 

 has been calculated, from the average size of its component cells, 

 that such a plant must have contained at least forty-seven 

 thousand million cells, so that they must have been developed at 

 the rate of rveaxly four-thousand millions per hour, or more than 

 sixty-six millions per minute. Another illustration of the rapid 

 production of cells is afforded us in arctic and alpine regions, 

 where it frequently happens that the snow over an extensive 

 area is suddenly reddened by the cells of the Ked Snow-plant 

 (Protococcus nivalis) {figs. 1 and 2), Again, it may readily be 

 ascertained that in a favourable growing season, many stems will 

 increase three or four inches in length in twenty-four hours, and 

 the Agave or American Aloe {Agave americana), when flowering 

 in our conservatories, has been known to develope its flower-stalk 

 at the rate of at least a foot in a day, and in the warm climates 

 where it is indigenous, as in the Mauritius, it will grow at least 

 two feet in the same period of time. Leaves also in some cases 

 develope very rapidly ; thus Mulder states that he has seen the 

 leaf of Urania speciosa lengthen at the rate of from one and a 

 half to three and a half lines per hour, and even as much as from 

 four to five inches per day. In all these cases of rapid growth 

 in size, it must be remembered that the increase is due not only 

 to the formation of new cells, but also to the expansion of those 

 previously formed. 



2. Absorption and Transjnission of Fluids. — The cell -wall of all 

 young and vitally active parenchymatous or prosenchymatous 

 cells is porous and readily imbibes fluids, and we find, accordingly, 

 that liquid matters are constantly being absorbed and trans- 

 mitted through such cells. The power which thus enables cells 

 to absorb and transmit fluids, is called osmose. This physical 

 force, as will be afterwards sho'wai, is a most important agent in 

 plant-life, for by its agency plants are enabled, not only to absorb 

 crude food by their roots in a fluid state, but also to transfer it 

 upwards, from cell to cell, to the leaves and other external organs, 

 for the purpose of being elaborated by the action of light and 

 air. It is, moreover, by a somewhat analogous process {diffusion 

 of gases), that the cells on the surface of plants are enabled to 

 absorb and transmit gaseous matters. 



Osmose may be explained as follows : — "Wlienever two fluids 

 are separated' by a permeable membrane which is capable of 

 transmitting them, the tendency to equalisation between the 

 two, from the formation of a current in both directions, will be : 

 modified by the action of the membrane, as well as by their 

 own rates of diffusion. This osmotic action may be easily ob- 



