SEAWEEDS AND THEIR ALLIES. 108 



two filaments. If the material is favorable, you 

 will be able by continuing such a comparison to 

 observe for yourself the successive stages in the 

 development of the zygospores.^ 



II. By cell-division. The nucleus undergoes a remark- 

 able series of changes, ending in its separating into two 

 new nuclei and the formation of a septum between them. 

 In this way a cell becomes divided into two " daughter 

 cells " which after attaining their full development divide 

 in the same way, the process continuing through a series of 

 generations.^ 



Spirogyra is one of the most abundant and widely 

 distributed of the green algae. It is always to be had, and 

 is one of the most satisfactory plants with which to begin 

 the study of the plant cell. Zygnema,, recognized by its 

 stellate chlorophyll bodies, and Mesocarpus, in which a flat 

 plate takes the place of a spiral band, are both often found 

 with it. All of these, particularly Spirogyra and Meso- 

 carpus, are capable of almost unlimited use in the demon- 

 stration of fundamental facts of vegetable physiology. 

 The student will do well to read carefully what is said 

 of Spirogyra in the laboratory manuals, and consult the 

 references in Arthur, Barnes, and Coulter's Plant Dissec- 

 tion, and the recent periodical literature. 



1 Cf. Strasburger, Practical Botany, p. 247 ; Sachs, Physiology o> 

 Plants, pp. 727, 728. 



2 Tor details of the process, including nuclear changes, see Strasburger's 

 admirable monograph, Ueher Kern- und ZelUheilung. Jena, 1888. 



