236 CRYPTOGAMS 



6. Zygospores, or oval masses which have formed out of 

 the united contents of the two cells. 



Notice the thickened wall which forms about the zygo- 

 spores. 



Note. Reproduction is effected in two ways in spirogyra: 

 (a) Asexually, that is, by pieces breaking off and continuing to 

 grow. (6) Sexually, that is, by the forming of zygospores through 

 the union of contents of neighboring cells. The process is known 

 as conjugation. The sex elements are different from those of 

 flowering plants in being alike in appearance. 



200. PLEUROCOCCUS 



This common alga is found on the north side of stone 

 walls, old fences, and tree trunks. It is usually more abun- 

 dant near the ground, but sometimes is to be found on old 

 roofs. It gives a green color to whatever it grows on, be- 

 coming brighter in damp weather. 



Object. To study the structure of pleurococeus. 



Method. Procure bits of bark or old wood showing 

 green color, note powdery appearance on dry surfaces ; place 

 in a damp chamber where they will remain moist. What 

 change of color follows ? Set in a warm place, preferably in 

 the shade. Remove some of the green material and mount 

 it in a drop of water. Examine first with a low power and 

 later with a higher (" objective) noting: 



1. The shape of the cells. 



2. Dividing cells as shown by pairs and clusters which 

 are still united. 



3. Can you distinguish between the cell wall and cell 

 contents? 



4. Can you determine the number, shape, etc. of chloro- 

 plasts? 



