LECTURE XXXV. 



IRRITABILITY AND MOBILITY OF PROTOPLASMIC STRUCTURES. 



The phenomena to be described here have ah-eady been in part superficially 

 examined in Lecture VI, but only a few of the chief points, were there touched upon ; 

 this was necessary in order to give those still unacquainted with the nature of the 

 vegetable cell an approximate idea of the nature of protoplasm. What has there 

 been said may therefore serve as an introduction to the present subject, and I may at 

 once pass on without preliminary explanations to the subject itself. Here also there 

 is no lack of bewildering variety in the phenomena, and this affects the treatment of 

 the subject the more, since -we are as yet by no means in a position to refer the 

 irritabilities and movements of protoplasm to any general principle whatever. At 

 present it is scarcely possible to speak of a comprehension of the processes from the 

 point of view of mechanics, indeed in many cases the merely sensible apprehension 

 of the processes, which are for the most part microscopical in range, presents great 

 difficulties even to a trained eye. Since in these lectures I avoid on principle entering 

 into difficult questions with discursive and technical discussions, and while with 

 respect to this point I refer to the notes at the end of the lecture, I shall select from 

 the variety of phenomena only a series of such as we may regard as different types. 

 This will perhaps best conduce to the reader obtaining a clear idea of the matter. 



The protoplasmic structures of which the movements and irritability are particu- 

 larly conspicuous to the observer are the swarm-spores of many Algae and some 

 Fungi already occasionally referred to, and the antherozoids (also in other respects 

 similar) of the Mosses and Vascular Cryptogams. Swarm-spores are naked, sharply- 

 defined protoplasmic bodies, usually of the shape of a fowl's egg : the larger thicker 

 half, in the case of those Algae which contain chlorophyll, being green, the anterior 

 narrower moiety colourless. Deviations from this common form, however, appear in 

 various degrees : the size also differs extremely, the swarm-spores of some Vaucherias, 

 for example, being visible in a good light even to the unaided eye of a sharp observer, 

 while the smallest forms can only be seen with high powers of the microscope. 

 Between these extremes are found all possible degrees of bulk. It may be noted 

 by the way, though it need concern us no further here, that many swarm-spores are true 

 sexual organs, while others are not. Some of the minute organisms belonging here 

 are during their whole period of life in oscillatory motion ; in others this motile stage 

 is very soon over, they become fixed to some body by the end which swims forwards, 



