VEGETATION TYPES. 495 



sive cold by the maturity and death of their aerial stems and 

 leaves. The underground stems and the roots are thus pro- 

 tected from the great loss of water which would result should 

 the aerial organs retain their power of transpiration. They are 

 further protected by being covered by soil, humus, leaves, etc. 

 In such plants as trillium, Indian-turnip (or jack-in-the-pulpit), 

 blood-root, spring-beauty, Solomon' s-seal, etc., the underground 

 stem is a thick rhizome or corm, and contains an abundance of 

 food so that the flowers and leaves formed in late summer or 

 early spring are quickly unfolded and appear as our early flowers. 

 The asters and goldenrods do not have such an amount of food 

 stored in their proportionately smaller underground stems. 

 Their aerial stem and leaves require a longer period of develop- 

 ment, and flowers appear from midsummer to late autumn. 

 In some of these plants sometimes the aerial leaves remain green 

 during the winter, but they form rosettes near the ground, and 

 are thus protected from great loss of water. 



959. Annuals and biennials. These, as suggested in para- 

 graph 938, may be considered tropophytes, since annuals every 

 year are carried through the period when the environment is 

 austere by their seed, while biennials are carried through in 

 alternate years by seed and by the rosette type of leaf arrange- 

 ment close to the ground, and by the short subterranean stem of 

 the first year. 



960. Halophytic structures. Xerophytic jorms. The great 

 body of partially submerged vegetation of the salt-marsh and 

 other shallow saline waters is furnished with xerophytic habit 

 and structures. The roots growing in soil saturated with 

 highly concentrated salt solutions absorb water slowly, and con- 

 sequently the aerial portions of the stems, as well as the leaves, 

 must be able to retard loss of water. This is brought about by 

 modifications of stems and leaves similar to those growing in 

 arid regions. These modifications are reduced leaf surface, 

 accompanied often by a thickening of the leaf, or an increase in 

 mass, in proportion to exposed surface, thickening of the epider- 

 mis, thickening of the cuticle, a lessening of the intercellular 



