INTRODUCTORY. 7 



act as exciting causes or stimuli : this being so, it must be 

 concluded that the movements are the result of exciting 

 causes existing within the organism itself, and therefore the 

 protoplasm is to be regarded as being automatic, that is, as 

 giving rise to the internal stimuli which cause the contraction 

 of the cilia. 



Although there is no reason to believe that stimuli acting 

 from without cause the movement of the zoospores, yet there 

 is evidence to shew that they may modify it. It has been 

 found by various observers that the movement of the zoo- 

 spores is affected by light : when exposed to light, the 

 zoospores arrange themselves so that their long axes are 

 parallel to the incident ray, and they move either towards 

 or away from the source of light according to circumstances. 

 It is evident, therefore, that the protoplasm is sensitive to the 

 action of this stimulus, and it will be shewn hereafter that 

 other external stimuli may also affect the protoplasm of the 

 cells of plants. This sensitiveness to the action of external 

 stimuli may be expressed in a general form by attributing 

 to the protoplasm the fundamental property of irritability. 



The protoplasm of the zoospore of Haematococcus is en- 

 dowed, then, with the following fundamental properties, in 

 addition to those which have been already enumerated with 

 reference to the Yeast-plant : 



5. it is contractile, as evidenced by the movements of the 

 cilia : 



6. it is automatic, in that the exciting cause or stimulus 

 which produces the contraction originates in the organism 

 itself : 



7. it is irritable, inasmuch as the movements of the or- 

 ganism may be modified by the action of external stimuli. 



These are, so far as can be ascertained, the fundamental 

 properties with which the protoplasm of the cells of plants 

 may be endowed, and in virtue of which it performs the func- 

 tions which together make up its life. Of these properties, 

 some, such as those of absorption, metabolism, and excretion, 

 are found to be possessed by the protoplasm of all living cells, 

 whereas the others do not appear to be so universally dis- 



