10 A PRIMER OF BIOLOGY 



which latter is filled with sap. A chemical analysis 

 of the sap shows it to be composed mainly of water, 

 but to contain also variable quantities of salts and 

 gases in solution, such as might have reached it from 

 without, and also of certain other substances not 

 derived, at least directly, from the exterior, which, 

 since they are always found in association with living 

 or dead organisms, have been termed organic com- 

 pounds. Since the plant 

 grows and, as we shall see 

 presently, multiplies, it is 

 obvious that the organic 

 material must gradually 

 increase in amount, and the 

 only sources of supply of 

 __ the raw materials required 



FIG. 5. Vaucheria. (A) For- for the manufacture of such 

 mation of a zoospore at organic substances must 

 the apex of a vegetative necessarily be the soil, the 

 branch (x 25). (B) A fully water an d the air. These 

 developed zoospore (x 50). materials must further 



(AfterOltmanns.) 



undergo certain changes in 



the plant to make them of such a nature that they 

 may be assimilated by the protoplasm the only 

 living part of the mechanism. Into these changes 

 we need not at present inquire, though, later on, 

 we shall find that the green particles, which are known 

 as chloroplasts, play a very important part in the 

 process. At present it will be sufficient for us to 

 recognise the fact that Vaucheria must absorb certain 

 materials from without and transform them into 

 pabulum, which it uses for the increase of its body 

 and for other purposes. It is to this series of 

 phenomena that we give the name nutrition. 



At certain times and under certain conditions, the 



