484 



LECTURE XXIX. 



Algae, as Pediastrum and CohochcBie scutata, may be looked upon, so far as their 

 relations of growth are concerned, as representing a thin transverse disc cut out of 

 a radially constructed shoot-axis. 



The ordinary case, however, is that of polar structure — i. e. the external and 

 internal organisation exhibit a peculiar arrangement along an axis of growth, in such 

 a way that all the relations of organisation are directed from the base towards the apex. 

 This is very evident, for example, in almost all shoots, where it is possible to see with the 

 greatest clearness even on small pieces which is the anterior and which the posterior 

 end, or which is the base and which the apex. Doubt could only exist on this sub- 

 ject, in the absence of exact investigation, 

 in the case of roots deprived of the apex. 

 The polarity to which I here . refer thus 

 indicates a progressive differentiation in 

 the direction of the longitudinal axis, 

 which may be compared to a certain 

 extent, at least so far as it concerns the 

 innermost nature — the capacity for reac- 

 tion—to that of a magnet. 



The polarity appears more sharply ex- 

 pressed, however, in the development 

 of a new individual from a spore or 

 from an, oospore, where a distinct con- 

 trast is at once observable between the 

 root-end and the shoot-end. This is the 

 case in the simplest and most elegant 

 form with the ovoid swarm-spores of many 

 Algae. The narrower colourless end, which 

 is anterior and provided with cilia during 

 the swimming movement, already indicates 

 the root-pole even before germination; 

 while the other end, green and thick, 

 may be distinguished as the shoot-pole. 

 The swarm-spore subsequently clings by 

 means of the root-pole to a solid body, 

 and the root-like organ of attachment is developed there, while the free shoot-pole 

 dfevelopes into a simple or branched shoot. This is not the place to go into many 

 other cases, and I may therefore at once refer to the behaviour of the young embryos 

 of the Vascular plants. Here also, as in the case of a swarm-spore, the con- 

 trast between root-pole and shoot-pole makes itself evident at an early period, 

 and when the embryo is to a certain extent further developed, especially in the 

 ripe seed in the case of the Phanerogams, it already actually consists of a root 

 at the one end and of the first shoot at the other. That these two poles, which 

 make their appearance even in the fertilised oosphere, subsequently come to act as 

 root and shoot, and have an importance which is not merely something external 

 and formal, but which dominates the whole being of the plant, follows not only 

 from the diflferent relations of growth of root and shoot, but equally from the 



Fig. 320.— longitudinal section through the grain 

 and embryo of the Wheat, only the lower part aza 

 being shown. C lower part of endosperm, f primary 

 root, g its root-cap; cc the so-called scutellum or ab- 

 sorbing organ of the embryo. 



