EXTERNAL CAUSES OF GROWTH AND FORMATION. II 319 



as large as those of plants grown in dry air and soil. Thus the outlines of organs 

 become less irregular in damp atmospheres (Kohl, 1886), that is to say the projec- 

 tions from the leaf become less marked, the ribs on the stem tend to disappear, 

 and the development of hairs becomes reduced. The anatomical differences 

 are even more remarkable. Increased transpiration tends to thickening of the 

 cuticle and induces the development of collenchyma and sclerenchyma, the 

 vessels are larger and more numerous, and abundant palisade parenchyma is 

 formed in the leaf. Critical researches are still required, however, to deter- 

 mine for us how far the observed results are to be put down simply to the dif- 

 ference in the amount of water present in the plant, and how far to the differ- 

 ences in the rate of transpiration ; if the latter factor be the important one it 

 will be necessary in the next place to find out whether the giving off of water 

 as such acts as a stimulus, or whether the supply of nutrients which stand in 

 close relation to transpiration is of greater significance. 



When plants which normally live in dry regions are cultivated in a moist 

 chamber very remarkable results are obtained. Lothelier (1893) found that 

 the formation of thorns was inhibited in a very damp atmosphere ; for example, 

 Berberis developed leaves in place of thorns and Ulex developed ordinary leafy 

 branches. Goebel (1898), who repeated these experiments, was not, however, 

 able to confirm these results entirely ; he was able to observe retardation, but not 

 complete inhibition, of thorn formation. Still more remarkable results were 

 obtained by Brenner (1900) from succulents. Fig. 89 (p. 305) shows the habit of 

 Sempervivum assimile, at I under normal conditions, and at // after lengthened 

 culture in damp air. As transpiration becomes weak the elongation of the 

 internodes destroys the radical rosette but later on a new rosette is formed. 

 The leaves grow more vigorously on their upper sides and hence bend towards 

 the ground in arches ; at the same time they become markedly thinner. This 

 diminution in thickness, the resolution of the rosette (which has been observed 

 in other plants under the same conditions by Wiesner (1891)), and finally certain 

 anatomical alterations, e. g. the bulging out of the epidermal cells, were cor- 

 rectly considered by Brenner as adaptations to aid transpiration. 



Many of the alterations in form and anatomical structure observed to take 

 place in plants grown in damp air remind us of those which occur in plants kept 

 in darkness. Sempervivum assimile, for example, loses its rosette form in dark- 

 ness and its leaves are markedly smaller. Increase in dampness of the atmo- 

 sphere is, however, very often associated in nature with diminution in light, and, 

 conversely, strong insolation is often mostly associated with more active transpi- 

 ration. In nature it is very rarely that we get a sitigle factor determining the 

 shape of the plant. If we find, for example, that subterranean branches are 

 different anatomically and morphologically (Costantin, 1883 and 1886) from 

 aerial branches, we are led to believe the true reason lies not only in differences 

 in illumination, but also in the amount of water present in the medium, and 

 possibly even in the fact that such branches are in intimate contact with soil 

 particles. Similarly, the characteristic structure of water-plants is concomitant 

 not only with retardation of transpiration, but also with the changed relationship 

 to light, oxygen, carbon-dioxide, &c. According toMAcCALLUM(i902), the active 

 cause of the aquatic form of Proserpinaca palustris is due entirely to retardation 

 of transpiration ; if transpiration be compensated by the osmotic absorption of 

 a concentrated salt solution, typical aerial leaves arise. We await confirmation 

 of these results. [The studies of Burns (1904) show that this phenomenon is 

 not so simple as it seems.] Similarly, several factors contribute to the forma- 

 tion of the alpine (Bonnier, 1895) and the halophytic (Schimper, 1891 ; Stahl, 

 1894) types of plant life. It is impossible for us to enter into further detail 

 here, so we will content ourselves with noting that the structure of the plant is 

 not predetermined once and for all, but that it is capable of being modified by 

 external conditions. 



