LEAVES 



593 



sap becomes relatively dilute. The palmella form can be produced 

 also by chemical stimulation, notably by toxic salts {e.g. salts of 

 copper, lead, and silver), and by solutions of bog water, where the low 

 concentration excludes osmotic pressure as a causative factor. 



So far as known, other algae usually react to changes in the concentration of the 

 medium after the fashion of Stigeoclonium, but the data are scanty. The fresh- 

 water alga, Mougeotia, when grown in 

 salt water for a time, becomes so 

 thoroughly " accommodated " to the 

 new environment that death ensues if 

 it is transferred suddenly to fresh water; 

 probably such "accommodation" con- 

 sists in increased concentration of the 

 cell sap. Growth in media of low 

 concentration causes the marine alga, 

 Cladophora trichotema, to become more 

 slender, while thickened cell walls re- 

 sult from an increased concentration. 

 Batrachospernmm, when grown in 

 weak light, develops only the embry- 

 onic or juvenile stage, long known as 

 the separate genus Chantransia. The 

 reactions of Stichococcus appear to be 

 the reverse of those of Stigeoclonium, 

 low concentration inducing the development of isolated spherical cells, while high 

 concentration induces the development of filaments of elongated cells, once referred 

 to the genus Rhaphidium. One of the fungi, Basidiobolus ranarum, reacts much 

 after the manner of Stigeoclonium, increased concentration (and also chemical stimu- 

 lation) resulting in shorter cells with thicker walls and in divisions in various planes. 

 Mucor and other fungi produce yeastlike cells through chemical stimulation. 



Form variations in amphibious plants.- — The phenomena. — -No plants 

 show greater variations in leaf form and structure than do amphibious 

 plants, which may be subject alternately to submergence and to desic- 

 cation. For example, the mermaid weed, Proserpinaca palustris, has 

 almost entire, lanceolate air leaves, and finely pinnatifid water leaves, 

 larger in outline, though of less weight (figs. 86i, 862-864). The vari- 

 ations of Radicula aquatica, one of the cresses, are very similar, and 

 even more striking, since the air leaves are quite entire, while the water 

 leaves may be twice or thrice pinnately dissected. In the water parsnip, 

 Sium cicutaefolium, the early radical leaves are much dissected, while 

 the later leaves are simply pinnate. In various buttercups there are 

 similar form changes, involving palmate rather than pinnate leaves (figs. 



862 863 864 j 



Figs. 862-864. — Leaves of the mermaid 

 weed {Proserpinaca palustris): 862, an air 

 leaf; 863, a water leaf; 864, a transitional 

 leaf. 



