222 PART III. VEGETABLE PHYSIOLOGY. 



organisms, and Protococcus, in one stage of its life history, 

 Pandorina and Volvox move by means of cilia the same as the 

 Infusoria. 



As regards irritability, that which plants exhibit is, of course, 

 less in degree than that which, in the higher animals, rises into 

 sensibility and sensation, but it can hardly be doubted, from the 

 evidence before us, that it is the same in kind. 



In animals this property is mainly concentrated in a highly 

 specialized tissue called nerve tissue ; hence its phenomena are 

 strikingly evident, while in plants it is diffused through all the 

 living tissues, and is in most cases but feebly manifested. But 

 these differences do not hold when we come to compare the 

 lowest forms of animal life with plants. In the lowest animals 

 there are no nerve cells ; the property of irritability is diffused, 

 as in plants. The Dionaea, the Sundew and the Sensitive Plant 

 exhibit a degree of irritability which equals, if it does not exceed, 

 that shown by the lowest animal types. Moreover, every grada- 

 tion is observed between the irritability of a tendril or a radicle 

 and that shown by the higher animals. The conclusion there- 

 fore is irresistible, that the property is fundamentally the same 

 in animals and plants — that irritability is an endowment of all 

 living protaplasm. 



Respiration, also, which consists essentially in taking in oxy- 

 gen and throwing off carbon dioxide, is much less evident in 

 plants than it is in animals, though certainly it is no less real. It 

 is a less noticeable phenomenon in plants, partly because, being 

 less active organisms, they waste less rapidly than animals do, 

 and the respiratory process is consequently slower ; partly, 

 because it is not carried on by means of a special breathing 

 apparatus, as it is in those animals .with which we are best 

 acquainted, but more, perhaps, because in ordinary green plants 

 the process is masked in the daytime by the assimilative one, 

 which goes on at the same time. In the latter process, carbon 

 dioxide is consumed as food, in quantities larger than that 

 thrown off in respiration ; and the amount of oxygen which is 

 consumed in respiration is more than counterbalanced by that 

 set free in the work of assimilation. For this reason, it is diffi- 

 cult, in the daytime, to demonstrate the respiratory process in* 

 green plants. But at night, the assimilation of carbon dioxide 



