The Present Position of Palaeozoic Botauy. 165 



further bifurcation, there are thus two synangia, and tliree segments 

 in all, wliile, if both bifurcate, we have three synangia and four seg- 

 ments. The variation occurs in the middle region of the reproduc- 

 tive zone in the most vigorous plants, and is extraordinarily frequent. 

 In a private letter, Prof. Thomas informs me that he has "found 

 as many as 150 sporophylls of repeated dichotomy on a single plant, 

 and shoots also in which an actual majority of the sporophylls show 

 two or three groups of sporangia". It must be remembered that these 

 elaborated sporophylls occur under the most favourable conditions and 

 are in no way of the nature of monstrosities. Prof Thomas com- 

 pares them with the sporophylls of CJieirostrobus, where there are 

 normally three sterile segments and the same number of sporangio- 

 phores ; a still closer parallel may perhaps be found in the repeatedly 

 dichotomous sporophj^ls of SphenopJujlIuni majus (Fig. 3), with which the 

 agreement seems to be almost exact, except that in that plant there 

 are four sporangia in each group, whereas in Tmesipteris there are 

 two, or in exceptional cases three. In other instances observed by 

 Thomas the synangium acquires a relatively long stalk, with 

 somewhat pendulous sporangia, strongly recalling Bowmanites Bömeri}) 

 Some similar variations to those in Tmesipteris were found by the 

 same observer in PsiJofum. The repeatedly dichotomous sporophylls, 

 which are so frequent as clearly to fall under the head of normal 

 variations, certainly appear to be fatal to the idea of any near affinity 

 between Psilotales and the Lycopods, while they strongly support a 

 relationship to the Sphenophyllales rather than to any other group. 

 This relationship also explains the normally forked sporophyll of 

 Fsilotum and Tmesipteris; it may well represent the dichotomous form 

 of leaf so common in SphenophyJhim. If we take the anatomical 

 characters also into account, there can be no doubt that on the 

 whole of the evidence there is a good case for the Sphenophyllaceous 

 affinities of the Psilotaceae. The arguments on which the comparison 

 of this group with the Ophioglosseae was based, apply with far greater 

 force to the Sphenophyllales, and are supported by additional characters 

 sufficient to indicate real relationship rather than mere analogy. 



Prof Thomas considers that we are justified in including the 

 Psilotaceae in the class Sphenophyllales and in this he is followed 

 by Prof Bower in his latest work. If we were compelled to choose 

 between Sphenophyllales and Lycopodiales, I should certainly incline 

 to the former alternative, as expressing the nearer affinity, but the 

 differences between Psilotaceae and the Palaeozoic plants which have 

 hitherto constituted the class Sphenophyllales seem to me too great 



^) I imss over the cases in which the synangium is replaced by a leaf-lohe 

 of normal appearance, because these are evidently of a teratological nature. 



