INFLUENCE OF EXTERNAL STIMULI. THE MEDIUM 265 



not proved, that this divergence is caused in many plants directly by 

 environment. 



Thus Johow 1 states regarding Philoxerus vermiculatus, a West Indian 

 shore-plant of the family of Amarantaceae, that the leaves possess in 

 sunny spots a cylindric form, whilst in the shade, where the plant 

 sometimes, though exceptionally, develops, they are generally thin and 

 disk-like. As I have already pointed out 2 , the causes for the change of 

 form here are altogether indistinct, inasmuch as the substratum according 

 as it has a greater or a less amount of salt, as well as the different con- 

 ditions of transpiration in sun and shade, might be considered as having 

 effect as well as the direct insolation. 



Battandier 3 points out that some Algerian species of Sedum which 

 grow in moist spots, for instance, S. stellatum and S. tuberosum, have flat 

 leaves, while species likeS. rubens and S. Magnolii, which grow on dry spots, 

 have in the moist period of the year flat leaves, in the dry season cylindric 

 leaves, and that in S. Clusianum the leaves in cultivation have a ( tendency ' 

 to become flat. These observations however only show us that the con- 

 figuration of the leaf stands in relation to the environment, but they 

 do not tell us whether environment directly acts upon them. Most 

 of our woody plants form at the end of their period of vegetation 

 leaves different from those they produced at its commencement, that is 

 to say, scale-leaves, but the difference is not directly caused by external 

 factors. How the species of Sedum just mentioned behave can only be 

 ascertained after exact experimental cultivation. 



2. HALOPHILOUS PLANTS. 



The majority of the higher plants are unable to grow in a soil which 

 contains more than 0-02 per cent, of chloride of sodium. The vegetation of 

 the seashore or of salt marshes is therefore relatively poor in species. Of 

 plants which are able to grow in such places the majority are distin- 

 guished by peculiarities of form which occur otherwise in the inhabitants 

 of dry areas such for example as succulence of the leaves or shoots, 

 reduction of the leaves, as in species of Salicornia, and other features 

 which impede transpiration 4 . These features are primarily connected 

 with the fact that the absorption of water from the soil is made difficult 

 by the presence of salt, and that water evaporates with greater difficulty 



1 Johow, in Pringsh. Jahrb. xv. p. 305. 



2 Goebel, Pflanzenbiologische Schilderungen, i. p. 31. 



3 Battandier, Quelques mots sur les causes de la localisation des especes d'une region, in Bull, de 

 la Soc. Bot. de France, 1887, p. 189. 



4 See the description of the peculiar leaf-construction in Acantholippia and Niederleinia in my 

 'Pflanzenbiologische Schilderungen,' ii. p. 13. 



