88 EMBRYOGENESIS IN PLANTS 



sometimes with a vascular strand, while Tmesipteris has first of all 

 similar scale leaves and later normal vasculated photosynthetic ones. 

 The possibility that the ancestors of the Psilotales were root-bearing, 

 leafy plants cannot therefore be completely ruled out. Manton (1950) 

 has shown that both PsiJotum and Tmesipteris have high chromosome 

 numbers and considers that these genera may be 'the end-products of 

 very ancient polyploid series.' 



Holloway has shown that considerable irregularities may be found 

 in the early segmentation pattern, Fig. 19b, f. These show considerable 

 departures from Errera's law. It may be that these embryos are 

 degenerating or abortive and have an abnormal distribution of meta- 

 bolites. Whether the anomalous histological condition is due to 

 environmental factors, to the simultaneous development of several 

 embryos on the same prothallus, to the nature of the nutrients supplied 

 by the prothallus, or to genetical factors, cannot be decided on the 

 evidence thus far available. Comparable irregularities are known in 

 other groups, e.g. the Equisetales. 



The normal embryogeny of the Psilotales would appear to afford 

 a model of what the development of a simple vascular plant might be 

 expected to be like: i.e. exoscopic embryogeny, the foot region in 

 contact with the source of nutrition, the organisation of a distal 

 formative region, and the growth and differentiation of a leafless and 

 rootless vasculated axis. It may be that the embryogeny of Rhynia, 

 Psilophyton, etc., was not unlike that of Psilotum, but information is 

 lacking. According to general phylogenetic theory, all classes of 

 vascular plants may have sprung from plants of psilophytean affinity, 

 or have passed through a psilophytean stage. It will, therefore, be of 

 interest to see to what extent the embryonic developments in other 

 classes of pteridophytes and seed plants correspond to the Psilotum 

 type of embryogeny. As we have seen in Chapter V, the bryophytes 

 have points in common with the Psilotales in respect of their early 

 embryogeny. 



In the Psilotales there is no suspensor — an organ found in the 

 embryogeny of some other pteridophytes considered to be primitive. 

 Lang (1915) pointed out that embryos without suspensors were prob- 

 ably derived from forms with suspensors. This raises the interesting 

 question as to whether the Psilotales have lost their suspensor in the 

 course of descent or whether that organ never was present in their 

 embryogeny. 



EQUISETALES 



In Equisetum the germinating spore divides by a curved wall into a 

 large and a small cell, the latter growing out as a rhizoid. Polarity is 



