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, 

 Stigeodonium, 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 concentration of the 



FIGS. 858-860. Variation 

 in Stigeodonium: 858, the 

 palmella form, consisting of 

 isolated spherical (a) or oblong 

 (i) cells; c, d, vegetative repro- 

 duction by means of fission, c 

 showing an early stage in which 

 a dividing wall is formed, and d, 

 a later stage, just before the two 

 daughter cells separate ; 859, the 

 filamentous form, in which the 

 elongated individual cells cohere 

 in simple or branched chains; 

 860, a filament that has been 

 placed in a concentrated solu- 

 tion and is beginning to break 

 up into the palmella stage; note 

 the rounding of the cells and 

 their subsequent separation; 

 highly magnified. -After LIV- 

 INGSTON (drawn from a photo- 

 graphic reproduction). 



