INOS 7] FLOWERING PLANTS AND FERNS 19 
water, in all degrees between almost total submergence, and 
eases where only the roots are covered with water o1 are 
buried in a water-soaked soil. Water lilies illustrate the first 
condition; swamp growth, as sedges, willows, and cypress re- 
present the second. 
Xerophytes.—These represent the other extreme in water 
relation of plants. Dry sand flats and sandy beaches, dry 
open plains, bare rock surfaces, and parched desert areas, all 
present unfavorable conditions for plant life. Besides lack of 
water in the soil, the air is parched and dry, and the sunlight 
is usually intense. Most of the common plants cannot live un- 
der these hard conditions, and those that do, acquire specially 
adapted structures to meet the conditions. In desert areas, 
characterized by minimum water and maximum heat, so hard 
becomes the struggle for preservation of the life both of the 
individual and of the species, that only a relatively few species 
can subsist, and these become the monstrosities of the vegetable 
kingdom; as for instance, the Cactus group, the Yuccas and 
allies, the thorny acacias and greasewoods, and the weird Tum- 
boa of African deserts. 
The danger to desert plants are threefold: Too great 
loss of water from the plant tissues into the dry air; insuffi- 
cient absorption of moisture from the parched soil; and des- 
truction by herbivorous animals. To meet the first of these 
dangers the leaves of the plant, through which transpiration 
of water takes place chiefly, are greatly reduced in size or 
are entirely absent, and the whole plant is very much compact- 
ed and covered with a thick ecutinized epidermis. The second 
is met by developing an enlarged root system that penetrates 
deeply into the soil, assuming often enormous proportions. 
Also the plant body is often thick and fleshy with a great dev- 
clopment of water-storage tissue, which greedily absorbs mois- 
ture during a period of plenty to be used in sustaining the 
plant in time of drought. To escape destruction by desert 
herbivorous animals to whom these fleshy succulent bodies 
would be toothsome morsels, a spiny or thorny armature 1s 
