CHLOROPHYCEAE 



229 



with needles will make it look as if the whole method of preparation 

 were wrong. Parts of nets mounted whole are shown in Figure 52. 



For details of the formation of starch and for the finer details of the 

 development of zoospores and gametes, Hydrodictyon should be im- 

 bedded and cut. 



Pleurococcus. — This form, which is used everywhere as a labora- 

 tory type of the unicellular green algae, is found on the bark of trees, 



where it is more abundant on 



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f&l 





m:^ 



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D ^^ 



the north side and near the 

 ground. It is also found on 

 stones and fences, and in moist 

 situations generally. It is easily 

 secured in nearly all locahties 

 and at all seasons. 



The life-history of Pleuro- 

 coccus is variously described 

 in textbooks, but it is very 

 doubtful whether there is any 

 mode of reproduction except by 

 cell division. The zoospores and 

 gametes which are sometimes 

 described probably belong to 

 other forms which are occasion- 

 ally associated with Pleurococ- 

 cus, especially when growing in 

 very moist situations. The life- 

 history was examined very 

 critically by the great algolo- 

 gist, Wille, who not only con- 

 cluded that cell division is the 

 sole mode of reproduction but 



showed how investigators, even those relying upon cultures, had made 

 their mistakes. Wille's paper was published in 1913 in Nyt Magazin 

 for Naturvidenskaberne. 



A study of the living material is sufficient for any general course. 

 The bright-green cells, scraped off and mounted in a drop of water, 

 show the rather thick wall, the chromatophores, and usually the nu- 

 cleus. A drop of iodine will bring out the nucleus, if it does not show 

 already, and will also stain the pyrenoid, if the cell contains one. A 



Fig. 52. — Hydrodictyon: A, part of young net 

 with segments showing one pyrenoid and one or two 

 nuclei; B, parts of three segments with nucleus, n, 

 and pyrenoid, p, well differentiated; C, part of a 

 still older segment with nuclei more numerous than 

 the pyrenoids; D, part of a nearly mature segment 

 with nuclei much more numerous than the pyre- 

 noids. Fixed in the Chicago chromo-acetic-osmic 

 solution and stained in iron-alum haematoxylin. 

 X600. 



