GENERAL CONCLUSIONS I79 



first appeared in the Cambrian {Aldanophyton) or the 

 Silurian (Baragwanathia). 



The great appeal of the Telome Theory lies in its economy 

 of hypotheses and in the way it allows the whole range of 

 form of vascular plants to be seen in a single broad unified 

 vista. Yet, to some botanists, this unifying influence is 

 regarded as a dangerous over-simphfication, to be treated 

 with great suspicion. It is in its appHcation to the Lycopsida 

 that it is most open to criticism and, in our present state 

 of knowledge, rightly so. The American palaeobotanist 

 Andrews 2^ sums up his views in the following words: 

 *Zimmermann's scheme for the pteropsids, or at least some 

 pteropsids, has much supporting evidence; his concept for 

 the articulates may be valid, but we are only on the verge of 

 understanding the origins of this group ; his concept for the 

 lycopsids is, so far as I am aware, purely hypothetical.' 



Figs. 28T-V illustrate the Enation Theory of Bower^ 

 which suggests that microphylls are not homologous in any 

 way with megaphylls. According to this theory, microphylls 

 started as bulges from the surface of the stem, and then 

 evolved into longer and longer projections, at first without 

 any vascular supply, then with a leaf trace that stopped short 

 in the cortex of the stem and, finally, with a vascular bundle 

 running the whole length of the organ. The microphyll, 

 therefore, has evolved by a gradual process of enlargement, 

 rather than by progressive reduction, and for this theory the 

 fossil record does provide some support : Psilophyton repre- 

 sents the first stage in the process (Fig. 28T), Asteroxylon 

 provides an example of the intermediate stage, where the 

 leaf-trace stops short (Fig. 28U), while Drepanophycus 

 represents a later stage with the leaf-trace entering the 

 lateral appendage (Fig. 28V). 



Whether the Lycopsida evolved in this way, or in the 

 manner suggested by Zimmermann, the starting point is, 

 nevertheless, the same in both cases — a plant with naked 

 forking axes — and it has been customary to quote Rhynia 



