7 6 



Practical Plant Biology. 



crosses the border line separating unicellular and multicellular 

 plants. 



Volvox is found in fresh water, very often in the pools or ditches 

 of woods and forests. It has the form of a transparent, gelatinous 

 sphere approaching or even attaining half a millimeter in 

 diameter. It is of a diaphanous green colour. There are few 

 who would not be impressed by the unique beauty of its appear- 

 ance. Microscopic examination shows us that in the healthy 

 condition the spheres of Volvox are motile, and that while they 

 steadily advance through the water in one direction, with one pole 

 constantly pointing forwards, they also rotate with a stately motion 

 round that pole in the line of their advance. Their rate of pro- 

 gression is about i mm. per second. It also is seen that the 



green colour of the sphere is 

 due to countless specks (they 

 vary in number from 200 to 

 50,000) situated near the sur- 

 face of the sphere and em- 

 bedded in a colourless, hyaline 

 material. A few larger green 

 bodies may be seen in many of 

 the spheres. Focussing and 

 manipulation of the light shows 

 that the central region of the 

 sphere is clear and apparently 

 filled with water. 



With a high power and re- 

 peated manipulation of the 

 light the outer surface of the 

 hyaline jelly which forms the 

 wall of the sphere may be 



made out and it will be noted that while the transition of the 

 jelly outwards into the water is abrupt and a defined surface is 

 visible, on the inner side this transition is gradual and the jelly 

 passes insensibly into the watery space within. The green bodies 

 embedded in the jelly, seen as specks with the low power, are 

 now easily made out to be cells. Each is a tiny pear-shaped mass 

 of protoplasm (say about 8//, long) with its narrow end just reach- 

 ing the outer surface of the jelly. The green colour is conferred 

 on each by the presence of green pigment located in the inner 

 end of the cell. The pigment is associated with a bell or cup- 

 shaped chloroplast like that of Chlamydomonas. In the substance 

 of the thickened, inner and closed end of the chloroplast a 



FIG. 13. Volvox aureus, asexual 

 sphere, x 80. Young spheres 

 are seen within the parent. 



