THE PSILOTALES 



The prothalli of both Psilotum (Fig. 231) and Tmesipteris are small cylindrical struc- 

 tures of a few millimetres or perhaps centimetres in length in the largest specimens, 

 living underground with the aid of a symbiotic fungus. Branching is dichotomous and 

 growth is by an apical cell as in the subterranean rhizomes of the sporophyte, which they 

 indeed closely resemble except for the numerous sex organs which they bear over their 

 surface. An additional point of resemblance met with in the Rangitoto material of 

 Psilotum was the presence of vascular tissue. This was found only in the largest prothalli 

 and it varied in extent. In the simplest cases the centre of the prothallus was occupied 

 by a group of elongated thin-walled cells free from fungus, as may be seen in Fig. 

 232 fl. In other cases an endodermis could be seen surrounding such cells, and 



Fig. 231. The vascular prothalli of Psilotum nudum from Rangitoto Island (see text), a. Intact prothalli 

 preserved in 70% alcohol, x 4. Inset natural size. b. A dividing cell from a prothallus from 

 this locality, after Manton (1942). x 1000. 



in yet other cases the central strand contained actual tracheids with lignified walls. 

 Some of these can be seen in the longitudinal view photographed by Mr Ashby from 

 one of HoUoway's original preparations reproduced in Fig. 233. The prothallial 

 vascular strand was liable to be interrupted at intervals by the invasion of the entire 

 central tissue by the symbiotic fungus, in which case the central cells remained un- 

 differentiated, and in the smaller prothalli, which formed the bulk of the collection, 

 they were quite absent. They were, however, obtained from different parts of the island 

 on each of Dr HoUoway's two visits, and could therefore not be dismissed as local mal- 

 formations, and it was their presence which prompted Dr Holloway to seek for a cyto- 

 logical investigation. 



The results of such an investigation carried out on material supphed by Dr Holloway 

 in alcohol after fixation in acetic-alcohol were that all sizes of prothalH from Rangitoto 

 Island were diploid, and that the sporophytes on the island to which they were no 

 doubt related were tetraploids. In addition to being tetraploids, which would not in 

 itself explain the presence of vascular tissue in the prothalh, a very characteristic and 



236 



