CHAP, c ORIGIN OF SPECIES 371 



sions, and alterations in the form or position of chloroplasts, and can 

 cause leaf-blades to assume another lie ; in like manner it can give lead 

 to the development of a type of morphological and anatomical structure 

 which is characteristic of the plants concerned, and which must be regarded 

 as being of use to them. Even the peculiar shapes of leaf-like Cactaceae 

 are mainly induced by light, as has been proved by Vochting ^ and 

 Gobel.- The etiolation of photophilous plants in darkness is presumably 

 to be regarded as a beneficial adaptation. The modelling action of light 

 is further illustrated by the differentiation of the vegetative organs of 

 Marchantia, and by the production of archegonia on the shaded face of 

 fern-prothallia. 



There are definite and constant differences between shoots in the soil and 

 shoots exposed to light, both in general and in a single species, or between 

 the structure of roots and shoots exposed to light. Costantin ^ cultivated 

 the same root or shoot in soil and in air ; and he proved that the manifold 

 external conditions left their different anatomical impresses on the organs, 

 and that the differences observed were identical with those characteristic 

 of plant-members normally living under corresponding conditions. 



The same truth is revealed by Lesage's experiments showing how 

 plants adapt themselves to a saline soil.** 



Experiments have also been conducted upon the action of heat on 

 plant-members. PriUieux and Vesque ^ show that when the temperature 

 of soil is raised the osmotic force of roots is increased, so that plants become 

 succulent, thus acquiring the water-reservoirs and considerable volume, 

 associated with a small transpiring surface, which help them to exist on 

 warm, dry, rocky soil, and the like. Thermal conditions may be responsible 

 for the increased production of wax that P. Nielsen and Raunkiar ^ have 

 observed in hot summers on the haulms of Hordeum, Triticum (section 

 Secale), and other grasses : this increase presumably depresses the 

 transpiration in a manner beneficial to the plants exposed to the changed 

 conditions. 



The anatomical and morphological characters of hydrophytes and 

 xeroph3rtes have already been discussed in this work. Costantin,' Schenck,** 

 Askenasy,^ Lothelier,^^ Dufour,^ Volkens,^^ Gluck,i^ and Gerschon,i* have 

 proved that different organs roots, stems, leaves, and hairs in a 

 single species undergo morphological and anatomical change, according 

 as these are developed in air or in water, or in dry or in moist air. They 

 also proved that the structural features thus induced were those generally 

 characteristic of terrestrial and aquatic plants, or of xerophytes and 

 hydrophytes respectively, or at least that development tended in those 

 directions. It is clearly a case of self-regulation when the intercellular 

 spaces become smaller as the factors inducing transpiration become 

 stronger, or the reverse. Certain species are very plastic ; for instance, 

 within a few weeks it is possible to change the land-form of Polygonum 

 ampliibium into the aquatic form.^'' 



Change in the food-supply causes a modification in the general form 



' Vochting, 1894. Gobel, 1908. ^ Costantin. 1883, sec also 1898. 



* See p. 219. ' See p. 124. Verbally communicated. 



' Costantin, 1883-5; see also 1908. ' Schenck, 18S4. Askcnasy, 1870. 



'" Lothelier 1890. " Dufour, 1887. " Volkcns, 1884. 



'' Gluck, 1905, 1906. '* Gerschon, 1905. " Massart, 1902, 



B b2 



