A treatise on the British freshwater algæ / by G. S. West.

  • George Stephen West
Date:
1904
Catalogue details

Licence: In copyright

Credit: A treatise on the British freshwater algæ / by G. S. West. Source: Wellcome Collection.

    39/402 page 13
    the division of a cell the pyrenoids usually divide equally. Some- times a pyrenoid in a well-nourished cell multiplies by division without any division of the cell or the cell-nucleus. In a badly nourished cell, the amylaceous portion first disappears and then the crystalloidal part. The pyrenoid is thus a store of reserve food-material, and it may arise quite spontaneously without the previous existence of pyrenoids in the cell. Almost all Algae are holophytes; that is to say, they are them- selves able to elaborate organic material from the mineral and other inorganic substances found in the water, or in some instances, in the atmosphere, in which they exist. The chlorophyll found in the chromatophores of the cells arrests certain rays of light, the energy of which is utilized by the living protoplasm for the con- struction of organic substance from the inorganic materials taken up. Algae absorb a relatively large amount of mineral food sub- stances, particularly nitrates, from the water in which they live. It has been assumed that the presence of nitrates in abundance is necessary for the prolific growth of Algae, but it is certainly true that these plants occur in quantity in water which is relatively poor in nitrates. Whipple and Parker1 state, as a result of experi- ments on the occurrence of small chlorophyll-bearing organisms in the waters of lakes, that the presence or absence of carbonic acid is one of the fundamental factors which influence the growth of Algae. The tropical Algae of the genera PJvyllosiphon and Gephaleuros are partial parasites, and a few have already been mentioned as symbiotic with other plants or even with animals. The growth of the thallus may be apical or intercalary. In many Algae it is by the repeated division of a single apical cell, or by a series of marginal cells, as in the expanded thallus of Coleo- chcete. In many of the filamentous Algae with intercalary growth all the cells of the thallus are menstematic and undergo division (e.g. Zygnemaceae, Ulotrichaceae). Vegetative multiplication, occurs in the unicellular forms by ordinary cell-division or fission, and in many of these plants it is the only method of increase. The division may take place in one direction only (e.g. Aphcinotliece, Glceothece, Stichococcus), in two directions in one plane (e.g. Tetrasporci, MerismopecLia), or in all directions of space (e.g. Gloeocystis, Gloeocapsa, and many others). 1 Whipple and Parker, in Trans. Amer. Micr. Soc. May 1902.
    No text description is available for this image
    No text description is available for this image
    No text description is available for this image