124 THE EVOLUTION OF CONTINUITY 



In the typical Continuously Zooidal Individual each 

 vegetative zooid is practically independent and its immediate 

 control is like that of the free zooid just mentioned, though 

 it is probable that one zooid may indirectly influence another 

 through the coenosarc. In the case of what we regard as 

 terrestrial derivatives of a primitive Continuously Zooidal 

 type — the Phanserogams — where the ancestral zooids are 



ton «f 

 iieit [>Ufs. 



C*l\ui f«rnud W 



Fig. 44. — The probable nervous system of the Phanerogam. 

 x, longitudinal section of sieve-tube in region of sieve-plate. 

 B, sieve-plate viewed from above, c, diagrammatic " node of 

 Ranvier " in medullated nerve-fibre. o, axis-cylinder ; m, 

 medullary sheath interrupted at node ; p, primitive sheath 

 thickened at node, (a, b, after Lowson ; c, after Key and 

 Retzius.) 



not discretely demarcated but develop in unbroken con- 

 tinuity, we expect to find some form of communicating 

 nervous system linking these up with one another. That 

 such a system is present there can be little doubt, but the 

 question is, Which of the continuous tissue-systems of the 

 plant represents it ? In all probability it is the sieve-tube 

 system, whose tubes traverse all regions of the plant, and 

 contain running down the centre an " axis " of albuminous 

 fluid, outside of which is a layer of un-nucleated protoplasm 



