Hollow ay. — The Prothallus and Young Plant of Tmesipteris. 



body of the prothallus shows its tissues to be composed of cells of uniform 

 size and shape, there being do differentiation of central long conducting 

 cells or of fungal zones such as are so well known in most of the types of 

 Lycopodium prothalli. The dense fungal coils occupy uniformly practically 

 all the cells in the central region, the epidermis and a zone three or four 

 cells in width immediately underlying it alone being free from these coils. 

 In the limbs of the larger prothalli this subepidermal layer sometimes con- 

 tains much starch. Moreover, meristematic activity sometimes shows itself 

 in these cells (fig. 19), though whether in connection with the storage of 

 starch or with the development of the sexual organs is not quite clear. The 

 mycorhiza extends uniformly right up through the length of the prothallus 

 to close behind the actual apex, keeping pace with the forward growth of 

 the latter. A series of transverse sections behind a growing apex shows 

 that at its uppermost limit the mycorhiza occupies only a narrow central 



Fig. 7. — Complete, branched prothallus, in which the branches are not 

 inclined to each other at angle but in opposite directions. X 10. 



Fig. 8. — Branched prothallus, one branch broken ; the other has 

 branched again in the manner described for fig. 7. X 5. 



core of cells, which gradually tapers off upwards, and that in these cells 

 the hvphae are more scantily developed. The fungal hyphae in these 

 growing regions of the prothallus are wholly absent from the cells which 

 surround the central core, this fact showing that when once the mycorhiza 

 has entered the prothallus in its earliest stages of development no further 

 infection is needed, but that the fungus extends upwards in a uniform 

 manner, keeping pace with the growth of the prothallus. The clear white 

 colour of the actual apex is, of course, due to the absence of the fungus 

 from its cells. In the older parts of the prothallus hyphae can often be 

 distinguished penetrating through the length of rhizoids and across the 

 outer layers of cortical cells, but it is probable (as is also considered to 

 be the case in other pteridophytic prothalli which are infected with a 

 mycorhiza) that this signifies no organic connection between the fungal 



