THE MATERIAL INCOME OF PLANTS 311 
Sap pressure and turgor. Turgor plays an important part also in 
' root pressure " (see p. 336), 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 on th^e 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. 
