U-2 Dr. F. 0. Bower [April 30, 



sporangia, stamp these plants as exceptionally primitive. Among 

 living plants the nearest of kin to them are clearly the Psilotacere, 

 a family which has long presented a problem in morphology and 

 classification. It comprises two living genera, Psilotum and Tmesip- 

 leris. Both genera are rootless. Their imperfect morphological 

 differentiation is shown by the fact that botanists are not yet agreed 

 whether their lateral appendages are to be held as truly foliar or not. 

 Psilotum is native throughout the tropics, and is represented by two 

 well-marked species. The commonest, P. triquetrnm,]ms upright and 

 shrubby aerial shoots, with radial construction and frequent bifurca- 

 tions. These spring from leafless underground rhizomes, profusely 

 bifurcated. They are covered with rhizoids, and contain a mycorhizic 

 fungus. On the lower part of the aerial shoots simple spine-like 

 leaves are borne, but towards the distal ends these are replaced by 

 forked spurs, between the prongs of which a synangium, usually with 

 three loculi, is seated. The aerial shoot is traversed by a vascular 

 strand consisting of xylem in the form of a hollow, many-rayed star 

 with sclerotic core, and branch-strands run out to the appendages. 

 The whole is covered by epidermis with stomata, and the cortex 

 provides the photosynthetic tissue. Tmesipteris is represented by 

 only one species, limited to Australasia. It grows usually among 

 the massed roots that cover the stems of tree-ferns, but sometimes 

 upon the ground. Its general form is like that of Psilotum, but the 

 underground rhizomes are longer and the appendages' larger, while 

 only two loculi are usually present in each synangium. Clearly the 

 form and vascular structure of these plants is generally like that of 

 the Rhynie flora. 



Till quite lately the Psilotacea? remained the only living Pterido- 

 phytes of which the life-cycle was still incompletely known. In all 

 the other groups the regular alternation of two generations had been 

 demonstrated ; the one is the prothallus, which is sexual, and the 

 other is the established plant, which is non-sexual. In the Psilotaceae 

 also the plant as above decribed is the non-sexual generation, but 

 hitherto the form or even the existence of the sexual generation 

 remained problematical. Since 1914 the prothalli of both genera of 

 the Psilotacene have been discovered, and their structure demon- 

 strated by Darnell-Smith, Lawson, and Holloway. And so the very 

 last of these life-histories has now been completed. It turns out 

 that the prothallus of the Psilotacea3 is similar in its general 

 characters to those of other archaic Peridophytes, being colourless, 

 and living in humus by means of fungal nourishment. In fact, these 

 plants conform in their life-cycle to what is seen in the Lycopods and 

 in the primitive Ferns. Analogy with the living Psilotaceaa makes 

 it highly probable that the Lower Devonian plants also showed 

 alternation. Though this has not been demonstrated for them, their 

 preservation is so perfect that even the delicate prothallus may yet 

 be revealed as the reward of further search. 



