. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . ntralplate. x 430 (modified from Kofoid). the instances of their occurrence suggest the influence of environmentalfactors in producing the mutations. Peridiniacese 73 If unfavourable conditions arise during cell-division, the process may besuddenly arrested, and undetached division-stages may be formed. Theseare not unlike certain of the presumed conjugation-states, and have some-times been mistaken for such. RESTING-SPORES. At the close of t
. Algæ. Vol. I. Myxophyceæ, Peridinieæ, Bacillarieæ, Chlorophyceæ, together with a brief summary of the occurrence and distribution of freshwat4er Algæ . ntralplate. x 430 (modified from Kofoid). the instances of their occurrence suggest the influence of environmentalfactors in producing the mutations. Peridiniacese 73 If unfavourable conditions arise during cell-division, the process may besuddenly arrested, and undetached division-stages may be formed. Theseare not unlike certain of the presumed conjugation-states, and have some-times been mistaken for such. RESTING-SPORES. At the close of the active vegetative period manyof the freshwater Peridiniacese are known to enter into an encysted conditionby the production of thick-walled resting-spores (figs. 500 and 54 E). Onlyone resting-spore is formed from a single individual. When the period of restis of some length, as in Peridinium aciculiferum, where it extends over nineor ten months (G. S. W., 09), the wall of the resting-spore is very resting-spores are formed at any time in some species when theenvironmental conditions become unfavourable, such as by a sudden change. Fig. 54. Ceratium hirundinella 0. F. Miill. A and B, three-horned forms; C and D, four-horned forms; E, three-horned form with resting-cyst. Z> is a ventral view; A—C, and Eare dorsal views. All x 200. of temperature or by alteration in the chemical constituents in solutionin the water. In most species the resting-spores are ellipsoid or ovoid,but in Ceratium hirundinella they are usually three-angled or four-angledand somewhat twisted, and each angle is furnished with a short spine(fig. 54 E). It is most probable that these resting-spores remain in the mudat the bottom of the water until the conditions are again favourable for theresumption of the active vegetative phase, but their development has not yetbeen observed. In some of the marine forms the protoplast becomes rounded off, secretesa wide gelatinous coat, to which
Size: 2241px × 1115px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1910, bookpublishercambridgeengtheuni
