THE STEM 65 



features. That is, the general type of machinery that serves one 

 species under given conditions comes to be assumed by all the species 

 living under the same conditions. As a result we are able to distin- 

 guish certain types of adaptation prevailing wherever certain sets of 

 conditions are found. The adaptation is seen in external form and in 

 internal anatomy. The types are the most marked where the condi- 

 tions are extreme. 



1. The Xerophytic Type is exemplified in desert plants. The ex- 

 treme condition is scarcity of water. The plant surfaces from which 

 moisture might be lost (leaf surfaces, particularly) are in these plants 

 reduced to the smallest limits. See, for example, Opuntia, in § 99, 

 which at maturity is without foliage leaves. A similar form is 

 exhibited by certain Spurges (Euphorbia) and Groundsels (Senecio), 

 quite unrelated plants. The internal anatomy is characterized by the 

 development of tissue for w^ater reservoirs, and of a thick waterj)roof 

 cuticular covering of the epidermis (see § 526). 



Between the extreme desert type and that of ordinary plants there 

 are all gradations. When leaves are present on xerophytic plants 

 they are likely to be leathery, or thick and succulent, or thickly cov- 

 ered with hair; the pores (§ .527) are sunken in the thick epidermis 

 and the leaf is often turned edgewise to light and heat. Xerophytic 

 characters are found in plants growing in dry situations in ordinary, 

 moist climates. 



Other causes besides dryness of soil and air may lead to scarcity of 

 "water in the plant, at particular times or in particular locations. In 

 temperate climates, for example, the winter brings frozen soil, and 

 consequent arrest of absorption at the root. Hence, the plants are 

 placed temporarily in xerophytic conditions, and most perennials meet 

 the emergency by the loss of leaves. So, also, the coldness of far 

 northern and high mountain soils produces a condition of drought, 

 ■with the resultant appearance of xerophytic characters in the vegeta- 

 tion. Root absorption may also be diminished by the presence of 

 salts dissolved in large quantities in the water about the root. Such 

 an effect is wrought in salt marshes, and on sea shores above the tide, 

 where the plants show characteristic xerophytic adaptations. Plants 

 fitted to life in such conditions are termed Halophytes. 



2. The Hydrophytic Type. — Submerged plants, and such as grow 

 largely submerged in fresh water, are in general characterized by a 

 thin epidermis, weak development of the framework, and large air 

 passages traversing the entire plant body. These interspaces allow 

 the penetration of air for respiration to submerged parts, as well as 

 give buoyancy to floating parts. For characteristic forms of the leaves 

 see §§ 130-1.35. 



2. The Mesophytic Type of structure is that of plants living under 

 ordinary conditions. The common tillage plants are INIesophytes. 



OUT. OF BOX. — 5 



