DEVELOPMENT OF PLANTS 163 



algae and illustrates many of the features in the life history of 

 the stationary forms. These plants often cause the green coat- 

 ing that appear upon the shaded or moist side of tree trunks, 

 fences and buildings. If a bit of this material is examined under 

 the microscope, it will be seen that the plants are rather spherical 

 cells provided with nucleus, cytoplasm, chloroplasts and well 

 marked walls. This latter feature is characteristic of stationary 

 algae and forms a sharp contrast to the delicate walls of the 

 motile forms. These plants increase in numbers by the forma- 

 tion of a wall through the middle of the cell. The two daughter 

 cells or plants grow to the size of the mother plant and repeat the 

 process in division by forming a wall at right angles to the old 

 wall. It will also be noticed that cell division may occur in 

 three planes. The plants adhere together for a considerable 

 period, or become detached shortly *after the division of the 

 mother cell. In this way they multiply with extreme rapidity 

 and so come to form extensive green coatings. It has been 

 noted in allied genera and reported in certain species of Pleuro- 

 coccus, also that changes of temperature and amount of water, 

 etc., will cause the contents of the cell to divide and form zoo- 

 spores which are of the same nature and behave in the same way 

 as in Sphaerella. Thus we see how changes of the environment 

 may cause these stationary plants to return to the motile condi- 

 tion of simpler types. It is interesting to note in this connection 

 that Livingston was able to change the stationary condition of 

 certain algae to the motile condition at will by diluting the 

 solutions in which they were living. Fig. 104, B, C illustrates 

 several other forms of stationary green algae that are common 

 in the drinking water and in ponds and streams. Interesting 

 forms for study may be obtained by tying a woollen sack to 

 a water faucet and allowing the water to run into it for several 

 hours. Invert the sack into a glass, washing off the material that 

 collects on the inside of the sack in the water of the glass. By 

 means of a pipette, a drop of the material that collects at the 

 bottom or sides of the glass or forms on the surface of the water 

 may be transferred to a slide and examined under a microscope. 



