4O Minnesota Plant Life. 



have the power of depositing in their interstices, silica, and it is 

 this which gives the polishing quality to the fossil powder. 



Some diatoms are provided with slender gelatinous stalks, by 

 means of which they attach themselves to objects under the sur- 

 face of the water, but many of them are free-swimming organ- 

 isms. The exact mechanism by which they swim has long been 

 a puzzle to students of the group, for they are not, like the switn- 

 ming cells of the green-slime which grows on flower-pots, pro- 

 vided with conspicuous lashes by means of which they roll them- 

 selves through the water. Most of them seem to have, how- 

 ever, extremely small apertures in their walls through which 

 the living substance probably protrudes itself and sets up an 

 agitation in the water, so that the tiny boat moves mysteriously 

 across the field of view of the microscope like some infinitesimal 

 electric launch. 



Red algae.. This group like the brown algae is essentially 

 marine and but few forms are found in the fresh waters of Min- 

 nesota. In rapidly flowing streams or under cataracts certain 

 kinds display their red bodies, appearing as delicate plumes a 

 few inches in length or as little pink or purple plates an eighth 

 of an inch or less in diameter. Their structure is more compli- 

 cated than that of any of the algae which have been considered. 

 They are supplied with egg-cells from which long cylindrical 

 protuberances are developed. The spermatozoids, unlike those 

 of the sphere algae, have no swimming lashes and are, therefore, 

 carried to the protuberances of the eggs by currents of water, 

 or by the ministration of aquatic insects, recalling in this latter 

 adaptation the extraordinary relation which exists between 

 insects and flowers. When, however, a little spherical spermato- 

 zoid comes in contact with the slender cylinder developed on 

 the egg-cell, it adheres and fuses and, as a result of its stimula- 

 tion, from the egg are thrust out branches which finally develop 

 spores, and thus the plant persists from one generation to 

 another. 



Some kinds of red algae are so faintly red that they would be 

 mistaken for brown algae, if color alone determined the classi- 

 fication. Such are certain rather stiff, wire-like plants, spar- 

 ingly branched and preferring for the most part the same rapid 

 foaming water which the easily recognizable varieties select as 



