Page:Popular Science Monthly Volume 63.djvu/247

Rh separated, the food-manufacturing tissue lying between the other two. These prostrate i)hints are all so small that no conducting system is needed.

So soon as a plant turns up into the larger and unoccupied space above the soil, the part which grows up cuts itself off from a direct supply of water and mineral food materials and exposes itself to greater loss by evaporation. The absorbing system of the part still in contact with the soil must be extended, the part above must be covered with material less permeable to water, and a conducting system which will supply the part above with water, which can come only from below, must develop. This we find in the erect mosses, and also in these cells which mechanically support the parts the weight of which is not wholly or directly carried by air and soil. The larger mosses, Polytrichum for instance, show these different tissues.

When a plant assumes the erect posture, its structure must correspond with its changed habit. The anatomical changes in man's body, which supposedly took place when he assumed the erect posture, have been explained by zoologists. Similarly there are changes in the bodies of plants which take on the erect habit of growth. These changes enable them to conform to the new relations and degrees of mechanical strains, the different relations to absorption and loss of water, the different relations to light, etc. The simpler, larger, erect plants, for instance the grasses, have worked out the relations of absorbing, protecting, food manufacturing, conducting, and mechanically supporting systems in very definite fashion. In these plants, absorbing and food-manufacturing systems are remote from each other, connected, however, by conducting tissues which carry the mineral salts and water needed for food manufacture, plus the amount of water which must inevitably be lost by evaporation, an amount constantly varying everywhere, but differing greatly according to situation, climate, etc. In these plants there must be the other conducting system, the one for distributing the food made in the leaves to all the living cells in other parts. Here we encounter, as in the ferns and their allies, which might equally well have been selected as illustrating these points, the double conducting system. The food-distributing system is found in all larger plants in which there are other living cells than those engaged in food manufacture. This is the primitive conducting system, the one first needed, as our consideration •of the larger aquatics showed. Only when absorbing and food-manufacturing tissues are remote from each other is another conducting system needed and developed, and the dimensions of this correspond with the volume of water to be carried to supply food materials and to make good the loss by evaporation.