Vegetative multiplication 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 Alga?, but it is certainly true 

 that these plants occur in quantity in water which is relatively 

 poor in nitrates. Whipple and Parker 1 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 Phyllosiphon and Cephaleuros 

 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- 

 chwte. In many of the filamentous Algae with intercalary growth 

 all the cells of the thallus are meristematic and undergo division 

 (e.g. Zygnemaceae, Ulotrichacese). 



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. Aphanothece, Gloeothece, Stick ococcus), in two 

 directions in one plane (e.g. Tetraspora, Merismopedia), or in all 

 directions of space (e.g. Gloeocystis, Glceocapsa, and many others). 



1 Whipple and Parker, in Trans. Amer. Micr. Soc. May 1902. 



