220 



ELEMENTS OF BOTANY. 



which Protoooccus is growing should he put In a shallow dish, loosely 

 covered with a pane of glass, to prevent drying up, and set in a sunny 

 place. 1 



267. Microscopical Examination of Protococcus.^ — Place a drop of 

 water containing Protococcus on a slide, lay on it a cover-glass, and 

 examine with a power 'of 200 or more diameters. Sketch with the 

 camera lucida several divisions of the stage micrometer alongside of one 

 of the largest cells, some of intermediate size, and one of the smallest.' 



Note the clearly defined cell wall, of cellulose, enclosing the proto- 

 plasmic contents, usually green throughout, sometimes red throughout, 

 sometimes of both colors. Do any cells show a nucleus like that in 

 Fig. 102 e, f,g,k,lf 



Test the cells with iodine for 

 starch. 



Note that the cell-contents in 

 many individuals has divided into 

 two parts, which become separ- 

 ated from each other by a cellulose 

 partition. The mode of division 

 is not unlike that shown in Fig. 

 190, but the cells in that figure 

 have not the distinct cell wall 

 that Protococcus has, while they 

 are covered with a layer of gel- 

 atinous material not found in 

 Protococcus. After the division 

 of a Protoooccus cell into two por- 



FlG.190. — A Unicellular Plant (PoJmo- 

 glma). (Greatly magnified.) 



Aj a single cell in its ordinary condition, 

 consisting of a mass of protoplasm col- 

 ored green by chlorophyll and sur- 

 rounded by a transparent gelatinous 

 envelope ; B, the cell-contents elongating 

 preparatory to multiplying by fission into 

 two portions ; C, the process carried a 

 step farther ; D, the two cells quite dis- tions, each may at once constitute 

 tinct but surrounded by a common ^ ^^^ ^gn ^^^1, ^ complete sac 

 gelatinous envelope ; E, each of the new - „ , , 



cells much enlarged and forming a °* cellulose surroundmg it, or 

 gelatinous envelope of its own. each of the halves formed by the 



first sub-division may break up 

 into halves again before the cell wall is formed around the new portions. 



1 If it is found impracticable to collect Protococcus for examination, the green, 

 powdery Plewrococcus found everywhere on the shaxly sides of trees or unpainted 

 fences will answer very well to show unicellular plants containing chlorophyll, and 

 to illustrate multiplication by cell-division. 



2 Slides permanently mounted and purchasable of the dealers (see Appendix C) 

 will answer for most of the microscopical examinations almost as well as the living 

 cells. 



' See Clark's Practical Methods in Microscopy, pp. 31-35. 



