THE MATERIAL INCOME OF PLANTS 311 



Sap pressure and turgor. — Turgor plays an important part also in 

 " root pressure " (see p. 330), by reason of which, under certain conditions, 

 water is forced by the cells of the cortex into the conducting tissues, 

 whence it may escape by filtering through the walls, or directly if these 

 are cut or broken. Further, it is probable that turgor is indispens- 

 able for the excretion of water and various solutes from superficial cells. 

 But this may be treated better in connection with the topic secretion 

 (P- 337)- 



4. THE PERMEABLE REGIONS OF ROOT AND SHOOT 



Plants and water. — Most if not all of the simpler algae and fungi, 

 many of the liverworts and mosses, practically all submersed plants, and 

 the young stages of even higher land plants are readily permeable to 

 water and to various solutes in every part of the body. In such case 

 they must grow in water or in very damp places. For, if water may be 

 readily admitted over the whole surface, it may be almost as readily lost 

 from the whole surface; it will evaporate whenever the air in contact 

 with any part of the surface is not saturated with water vapor, and 

 this is the usual condition. 



Terrestrial plants. — The earliest plants on the earth's surface, it is 

 likely, were aquatic; and in the course of time plants developed that 

 were adapted to temporary exposure oh the shore rocks or along the 

 beaches, then to longer exposure and drier ground, until the land finally 

 was occupied by plants which are so constructed that they can expose 

 a large part of the body continually to moist though unsaturated air. 

 The deserts even, with only a meager rainfall, are by no means barren of 

 vegetation, but support hosts of plants, which are able to secure the 

 scanty moisture from the soil and to avoid in the growing season ex- 

 cessive evaporation into the very dry and often very hot air to which they 

 are exposed. The prime requisite to terrestrial life is some means of 

 reducing the evaporation from aerial parts to an amount which can be 

 replaced by the water entering those parts of the body that remain in 

 contact with it. 



The root system. — The members of the higher plants constantly in 

 contact with water pertain chiefly to the root system. 1 Of the root sys- 



1 In some plants the underground stems and leaves (scales) are also in contact with 

 water, but they are almost impermeable to it, and hence may be neglected in this connec- 

 tion. 



