THALLOPHYTES 



13 



jelly-like mass; and Gloeotrichia (B), with its cells arranged 

 as in Nostoc, but showing that the cells are becoming dif- 

 ferent, so that the base and apex of the colony are not alike. 

 Gloeothece (like Pleurococcus) is found in bluish green patches 

 on tree trunks, fences, walls, etc. ; and Nostoc occurs as small 

 lumps of jelly in damp places. In these colonies the cells 

 (individuals) are held together mechanically by the mucilage, 

 but they seem to be as independent as if they were separate. 



In the case of other colonies, 

 such as the one shown in Fig. 5, 

 the cells are much more closely 

 related, being pressed against one 

 another so as to flatten the walls 

 that are in contact. Although 

 the cells of this colony (Oscil- 

 latoria) are for the most part in- 

 dependent of one another, as 

 shown by the fact that they 

 may break apart and live inde- 

 pendently, they work together in 

 , certain ways, notably in the char- 

 acteristic swaying and revolving 

 movements of the colony as a 

 whole, movements that have 

 given name to the plant. This interesting plant forms bluish 

 green slippery masses on wet rocks, or it occurs on damp 

 soil or freely floating on the water. 



When the individual cells of a colony work together in a 

 still more intimate way, the colony of many individuals be- 

 comes the individual of many cells. This many-celled in- 

 dividual is the third stage in the progress of the plant body, 

 and it is evident that there is no way of telling just when 

 a colony becomes such an individual. The three general 

 stages, therefore, are (1) the single cell, (2) the colony, 

 (3) the many-celled individual. All of the remaining 



FIG. 3. Glceothece: in the upper 

 left-hand corner is a single 

 plant (one cell), with its nu- 

 cleus, cytoplasm, and wall, and 

 also a covering of mucilage de- 

 veloped from the wall-material 

 (cellulose) ; in the other figures, 

 successive divisions are shown, 

 resulting in an irregular colony 

 of individuals held together by 

 the investing mucilage. 



