CHAP. VI HEAT 27 



prostrate forms are rare, and such forms are protected by succulence, 

 strong coatings of hair, and the hke> 



These famihar forms of growth have beyond doubt a common cause. 

 With the popular explanation that the plant wishes to ' bend before the 

 wind ', science cannot content itself. Probably the cause must be sought 

 in the difference of temperature of the air and soil at the time when the 

 shoots are developing. There are often to be met, growing side by side, 

 erect and prostrate individuals, e.g. of Atriplex, Salicornia, Suaeda, 

 and others on the northern coasts of Europe ; this fact denotes that the 

 decisive cause is no general factor prevailing at all seasons in a definite 

 place. Neither the wind nor its direction can be responsible, since indi- 

 viduals of one species growing on the same shore may have their main 

 shoots pointing in different directions, as may easily be observed on the 

 North-European coasts. The explanation must, apart from individual 

 pecuHarities, probably be sought in the different degree of heating of the 

 plants during their development above ground, and the consequent exe- 

 cution of thermotropic movements. Krasan ^ suggests that plants on 

 a warm soil, particularly in a climate with a warm atmosphere, acquire 

 vigorous erect shoots, but on a cold soil, and especially with an alpine 

 climate, prostrate ones. That psychrocliny in reality is responsible for 

 the espalier-like prostrate growth of plants in various cases, is confirmed 

 by the admirable investigations of Vochting and Lidforss.^ Henslow ^ 

 also expresses the view that thermotropism plays a part. 



On roads and soil frequently trodden down, prostrate forms, such as 

 Polygonum aviculare, are frequently found. Here the cause is perhaps 

 mostly strong vegetative heliotropism. 



2. Rosette-plants. Many herbs have their basal leaves more or less 

 liorizontally expanded in the form of a rosette ; even when they have 

 elongated rhizomes or subterranean stolons their shoots, upon reaching 

 the surface, become condensed. What factors are responsible for this is 

 scarcely known ; but presumably heat plays some part. Bonnier ^ has 

 experimentally demonstrated that great changes in temperature are 

 among the most eflicient factors in determining the character of alpine 

 plants ; those plants that were exposed to intense cooling action at night 

 acquired condensed stems, smaller, thicker, and harder leaves, and they 

 blossomed earher. Conditions of illumination exerted less influence. 

 Rosette-herbs occur in great numbers in temperate countries and are 

 particularly characteristic of sunny meadows which are covered by low 

 vegetation ; they are found in great numbers in arctic countries and on 

 high alpine situations on open grassy or rocky expanses, yet they occur 

 in still greater quantity on the meadows of lowlands, but rarely in forests.'' 

 In hot dry climates rosette-plants are less common, some occurring 

 only on moist soil," some others being confined to hot dry spots and, 

 in such cases, characterized by succulence (e.g. with succulent leaves, 

 Echeveria, Aizoon, Agave, and Bromeliaceae) or by other potent means 

 of protection against drought. 



3. Tufted (caespitose) growth and scrub-plants are common in climates 



' Vahl, 19046. ' Krasan, 1^82, 1884. 



" Vochting, 1898 ; Lidforss, 1902, 1906 ; compare C. Schroter, 1904-8. 

 * Henslow, 1894. " Bonnier, 1898. * Warming, 1901. 



' Meigen, 1893, 1^94 5 Vahl, loc. oil. 



