390 



Transactions. 



a medium-aged prothallus which is beginning to fork, and fig. 3 its basal 

 end. Fig. 5 is a median longitudinal section of the lower regions of a still 

 older prothallus, in which the gradual extension in width in a series of 

 gentle swellings is clearly seen. The latter prothallus had preserved the 

 unbranched carrot form unusually long (fig. 6), and was rather more 

 attenuated in form than usual, but it illustrates well the manner in which 



the Tmesipteris prothallus always in- 

 creases in girth. That shown in fig. 4 

 is more typical. In the latter the 

 first forking of the head had already 

 taken place, and the two large swollen 

 apices were in the act of forking 

 again. The shaded area in the main 

 body shows the extent of the fungal 

 distribution, the two apices being quite 

 clear. 



The prothalli vary a good deal in 

 both length and thickness. Measure - 

 nients of mature specimens show; d 

 variations in thickness from 0-3 mm. 

 to 1-25 mm., and an extreme length 

 of 18 mm. has been observed Gene- 

 rally speaking, it is the more attenu- 

 ated prothalli which show the greater 

 tendency to an extension in size by 

 a second forking of the apices, the 

 thicker individuals seldom appearing 

 to fork more than once unless one of 

 the first branches has ceased to grow. 

 The stouter prothalli frequently pos- 

 sess much-swollen heads. The apex 

 of the attenuated form was given in 

 longitudinal section in my previous 

 paper at fig. 21, while that of the 

 stouter form is shown in the present 

 paper in longitudinal section in fig. 7, 

 There is always a single apical cell of 

 the same form as is found throughout the life of the sporophyte. I have 

 found that serial sections of large thick prothalli may show the presence of 

 several fertilized archegonia and very young developing embryos in close 

 proximity to one another, but I have seldom found more than one plantlet 

 attached to a single prothallus. 



The close similarity in general appearance between the prothallus and the 

 very young sporophyte must be noted. This, of course, arises mainly from 

 the fact that the prothallus possesses an extended and branched chlorophyll- 

 less body with a fairly regular radial growth, and that the young sporophyte 

 consists at first of a simple branching rhizome devoid of appendages, both 

 being brown in colour and covered with the same long brown rhizoids. 

 It is sometimes quite impossible to be sure to which of the two a fragment 

 belongs until it is closely examined under the microscope. This similarity 

 is more marked in the case of Tmesipteris than in any other Pteridophyte. 

 The young adventitiously-produced plantlet of Psilotum is also similar 

 in appearance to the prothallus. 



Fig. 5. — Longitudinal section of the basal 

 end of the prothallus shown in fig. 6. 

 X 63. 



Fig. 6. — General longitudinal section of 

 a prothallus, showing its gradual ex- 

 tension in width, and also an attached 

 plantlet. X 12. 



and in transverse section in fig. 8. 



