78 Practical Plant Biology. 



Volvox evidently, the co-ordination must be of a more complicated 

 nature, for while the cilia of the cells immediately at the anterior 

 pole may act in a way comparable to a single cell of Chlamydo- 

 monas, those at the equator and at other parts of the sphere must 

 react differently to the light, and in order to produce the proper 

 propulsive and orienting forces which make the sphere move to- 

 wards the source, stimuli from the cells bearing stigmata must be 

 conveyed to every other cell and the responses must be so har- 

 monised and co-ordinated that the observed steady motion will 

 be produced. Probably the elaborate network of protoplasmic 

 fibrillae finds one of its functions in transmitting these co-ordinat- 

 ing stimuli. 



Just as in the case of Chlamydomonas, a moderate light stimulus 

 attracts Volvox. An intense beam of light, on the other hand, 

 repels it. When cut off from light the sphere wanders about ex- 

 hibiting a so-called spontaneity in its motions. This spontaneity 

 is probably to be attributed to infinitesimal stimuli, often of a 

 chemical nature, or its actions may be determined in part by the 

 previous history of its various constituents acting like the hysteresis 

 of colloids and producing effects which are often spoken of as 

 physiological memory. 



So far as is known the nutrition of Volvox resembles that of 

 Chlamydomonas. It obtains carbon by photosynthesis. The 

 evolution of oxygen and the growth of starch in the pyrenoids 

 tells of this process. Also its acquisition of nitrogen is from 

 nitrates. It is in fact a holophyte. Simultaneously with its con- 

 structive processes, destructive changes are taking place in its 

 cells. The chief of these is respiration and its waste products, 

 carbon dioxide and water are eliminated, while free oxygen is 

 absorbed. It is this aerobic respiration or oxidation of the carbon 

 assimilated in photosynthesis which provides the energy utilised in 

 ciliary motion and other vital processes. Other waste products 

 not so easily defined are eliminated by the action of the con- 

 tractile vacuoles. 



Volvox has both sexual and asexual reproduction. 



In most spheres of Volvox a number of smaller spheres are 

 seen enclosed. There are usually about eight, and they are dis- 

 posed round the posterior pole within the sphere. These en- 

 closed bodies vary in appearance according to the stage of their 

 development. In their earliest stage they are enlarged spherical 

 cells which resemble the other cells of the sphere, but owing to 

 their size protrude from the wall into the internal cavity of the 

 sphere. The protoplasm of each of these cells contains an 



