374 LECTURE XVI. 



previous rate of growth will be maintained for a longer or 

 shorter time before the organ accommodates itself to the new 

 conditions. The same is true of stimulation : a movement is 

 not immediately produced by the action of a stimulus, but 

 there intervenes between the action and the response a longer 

 or shorter " latent period," which is of course extremely short 

 when compared with the corresponding period in the response 

 to a change in the tonic conditions. 



The degree of general irritability is by no means uniform 

 among plants. We shall hereafter meet with the greatest 

 possible differences in this respect, some plant-organs being 

 scarcely at all sensitive to the action of any external agent, 

 whereas others are highly sensitive to all. Again, a plant- 

 organ is not necessarily equally sensitive to the action of 

 different agents ; we may perhaps most readily form a 

 satisfactory conception of this by ascribing to the organ a 

 " specific irritability " with regard to each agent, this specific 

 irritability being in some cases relatively considerable and 

 in others relatively slight. 



With regard to the distribution of irritability in a plant- 

 organ, the sensitiveness to the action of a stimulus may be 

 possessed equally by all parts, or it may be localised in some 

 particular part, or, again, it may be possessed in unequal 

 degrees by different parts. It is not necessarily the case that 

 the irritable region of the organ is also that part of it by 

 which the responsive movement is performed, but the irritable 

 and motile regions may be more or less widely separated. 

 When this is the case there must evidently be some means of 

 communication between them, so that the effect produced in 

 the irritable region by the action of the stimulus may be 

 transmitted to the motile region. This communication is set 

 up most probably by means of the delicate filaments which, 

 as mentioned in a previous lecture (p. 23), have been found in 

 many cases to connect the protoplasm-bodies of adjacent cells. 

 We will return to this subject and discuss it more fully in a 

 subsequent lecture. 



It will be convenient to classify the movements of plant- 

 organs in the manner suggested above, into, namely, the 



