LAND AND WATER PLANTS. 243 



seiDarated, 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. 



^Vhen a plant assumes the erect posture, its structure must cor- 

 respond 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 mechan- 

 ically 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 min- 

 eral salts and water needed for food manufacture, plus the amount of 

 water which must inevitably be lost by evaporation, an amount con- 

 stantly varying everywhere, but differing greatly according to situa- 

 tion, 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 illus- 

 trating these points, the double conducting system. The food-dis- 

 tributing 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-inanu- 

 facturing 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. 



