THE FUNCTIONS OF THE ORGANISM 13 



For reasons we shall afterwards appreciate, sensi- 

 tivity to external stimulus is not so observable in the 

 plant as in the animal ; still, even in Vaucheria, it 

 is capable of demonstration. Thus the zoospores 

 may be made to move in definite directions in response 

 to the stimulus of light. A bright light causes them 

 to move away from, a weak light induces them 

 to move towards, the source of illumination, while 

 the mucilage which escapes from the short swollen 

 branch containing the ovum attracts the sperm. 

 Many experiments have been performed of recent 

 years on Vaucheria (and on other plants equally low 

 in rank) which go to prove that these plants are 

 extremely susceptible to external influences, such 

 as running versus stagnant water, salt solutions of 

 varying strengths and chemical character, light and 

 darkness, &c., to which influences they respond in 

 various ways. 



Let us now consider an illustrative example from 

 the animal world, e.g., a frog. Obviously such an 

 animal is of relatively much higher grade zoologi- 

 cally than Vaucheria is botanically, but it is a familiar Frog, 

 and easily obtained organism, and illustrates certain 

 characteristic features of the animal kingdom better 

 than one of lower rank. 



From the nutritive point of view we notice at once 

 that the frog differs markedly from Vaucheria in that 

 almost all the materials which it absorbs are organic ; 

 it makes but little use of the minerals in the soil, and 

 does not employ the gases of the air at all as food 

 materials. We shall find later (p. 43) that this is 

 associated with the absence from its mechanism of 

 the chloroplasts which we have seen to be one of the 

 constituents of the filaments of Vaucheria. This 

 organic " food " material is moreover taken in by 



