IRRITABILITY. 419 



that all the cells are not capable of growth, but that certain 

 cells only can grow, and that these, stating the matter, in its 

 simplest form, are confined to the apex, constituting what we 

 know as ^.punctum vegetationis (p. 338). In a typical embryonic 

 Vascular Plant we find that the body presents a higher degree 

 of complexity, in that a punctum vegetationis is present at 

 each end of its axis of growth. In this case the body consists 

 of two well-defined members, each having its free apex and 

 punctum vegetationis, their common base being the plane along 

 which they are welded together. If, as is quite permissible, 

 we compare the body of a Fucus to a cone standing on its 

 base, the punctum vegetationis being at the apex, we must 

 compare the body of the Vascular Plant to two cones with 

 their bases in contact. These two members, as we shall 

 learn hereafter, possess different kinds and degrees of irri- 

 tability, and on this account we call them by different names; 

 the one we call the primary shoot, the other the primary 

 root. 



If, now, we observe the course of growth in these various 

 plant-bodies, we find in Oedogonium and in Fucus, that the 

 axis of growth is approximately a straight line. Similarly, the 

 axes of growth of both the primary shoot and the primary root 

 of the embryonic Vascular Plant are approximately straight 

 lines; their axes of growth form, in fact, one continuous straight 

 line ; but the growth in length of primary shoot and primary 

 root takes place in opposite directions along this straight line. 

 This opposition in the direction of growth of primary shoot 

 and primary root is a point of great importance in the subject 

 with which we are now engaged. 



We may now proceed to enquire somewhat into the 

 causation of these phenomena, in order to ascertain how far 

 they are the expression of tendencies inherent in the 

 plants. 



The body of an Oedogonium is developed from a zoospore 

 which is ovoid in form and moves by means of a circlet of 

 cilia surrounding the more pointed end which is hyaline and 

 uncoloured. As the zoospore swims, the pointed end is 

 directed forwards, and when it comes to rest the zoospore 



272 



