494 POPULAR SCIENCE MONTHLY. 



The two ends of the microscopic aqueous canals, in plants of the 

 ordinary sort, are the active imbibing root-hairs, above mentioned, and 

 the exposed surfaces of the aerial parts, chief of which are the leaves. 

 The lower terminal is buried beneath the soil from which it receives 

 its supply, and consists of a sac with a very thin and elastic wall, lined 

 with a delicate film of living protoplasmic substance. We may imagine 

 one of these cells many hundred times enlarged, its contents con- 

 sisting of a thin syrup, but slightly more dense than the liquid in 

 which it is immersed. As time passes, there is a flow inward of the less 

 dense liquid and an increase of the wall tension of the cell. This 

 tension might be observed by pricking the wall, when from the pin- 

 hole the liquid would spurt for some distance. The same pressure 

 might aid in the passage of the liquid from the sac to the one next 

 adjoining, and in that way a flow would be set up from the less to the 

 more dense cell contents. 



A homely and common illustration of this osmosis or membrane 

 diffusion is seen in the action of sugar upon ripe strawberries, the sugar 

 taking the thin juice from the cells and making a syrup that flnally 

 surrounds the berries. Place dried prunes in a dish of warm water, 

 and a similar exchange is demonstrated, but in this case the flow is 

 into the dry cell contents, and the prunes finally become plump. There 

 has been a transportation of liquid in both these instances, and it has 

 been from one cell to another through the whole necklace, so to say, of 

 many beads, from the surface to the innermost cell or vice versa, as the 

 case may be. 



Let us ascend a tall tree, figuratively, and study microscopically 

 the upper terminals of the lines of water-carriers. Here we find the 

 leaves in great numbers, presenting, possibly, acres of actual visible 

 surface to tile drying influence of the almost constantly changing air. 

 But if we note the exceedingly porous structure of a leaf, how one cell 

 touches its fellows at but few points and the bulk of the space is in- 

 tercellular, the actual surface exposed to the atmosphere is a hundred 

 times more than the naked eye reveals. As with the soil terminal, so 

 here the end of the transportation line divide up into a million parts. 

 In the former, each is for the reception of liquid; in the latter, they 

 are all places of unloading. The drying air sweeps over them, and 

 something of their contents is vaporized and is gone from the plant. 

 But this evaporation increases the density of the cell contents and were 

 there no reserve the tissues would wither, dry up and become dead, as 

 is the case when a branch is cut off or grass is mown in the meadow. 



If we apply the law illustrated in the dried prunes, it will be seen 

 that each surface cell in the loose pulp of the leaf is dependent upon 

 the next below it, and that, in honoring the draft upon it, is making a 

 physical call upon another, and so the line is established, like men 



