. A textbook of botany for colleges and universities ... Botany. LEAVES 591. forms always have been rigid and others always plastic. Quite apart from evolutionary considerations, the study of the cause of leaf form is important, because of its bearing upon the fundamental problems of plant behavior, and because of its relation to the role of leaves, including the advantages and dis- advantages associated with the different leaf forms in various habitats. Form variations in thalloid plants. — The variations in body form exhibited by algae and fungi are in many respects com- parable to those of
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. A textbook of botany for colleges and universities ... Botany. LEAVES 591. forms always have been rigid and others always plastic. Quite apart from evolutionary considerations, the study of the cause of leaf form is important, because of its bearing upon the fundamental problems of plant behavior, and because of its relation to the role of leaves, including the advantages and dis- advantages associated with the different leaf forms in various habitats. Form variations in thalloid plants. — The variations in body form exhibited by algae and fungi are in many respects com- parable to those of leaves, though some- what simpler, thus clearly meriting con- sideration here. In nature the alga, Stigeoclonium, exhibits two widely con- trasting forms: one, the palmella form, once thought to belong to the separate genus, Palmella, is common on moist bark and consists of relatively thick-walled spherical cells, which divide in any plane, and either cohere in colonies or become isolated (fig. 858); the other form is fila- mentous, the individual cells being elon- gated and relatively thin-walled, and di- viding in but one direction (fig. 859). It has been shown that if the filamentous form is grown in a medium of relatively high osmotic pressure, the palmella form is produced, the cells soon bulging out and becoming spherical, and later separating; subsequent divisions occur in all planes (fig. 860). On the other hand, the fila- mentous form is produced, if the palmella cells are grown in a medium of relatively low osmotic pressure. While only young palmella cells can grow into filaments, adult filament cells are capable of developing directly into palmella cells, contrary to the general rule that adult forms are not plastic. The filamentous form appears to be the more vigorous, probably because the low concentratioB of the Figs. 858-860. — Variation in Stigeoclonium: 858, the palmella form, consisting of isolated spherical (a) or oblong (i) cells; o, d^ vegetative repro-