XIV] 



THE MYCORHIZIC TYPE 



283 



ordinary cordate type. Whether this is anything more than a fanciful com- 

 parison must for the present remain an open question. Against it is to be 

 placed the fact that all the Ferns quoted are epiphytic except Schizaea, 

 which is a ground dweller, growing among Sphagnum, etc. It is a possibility 

 that the circumstances of epiphytic growth may favour, in Ferns not related 

 closely to one another, a similar form of prothallus, together with vegetative 

 propagation by gemmae. 



Fig. 274. Prothalli and sexual organs of Hehninthostachys zeyianica, after Lang, a, (^ = pro- 

 thalli seen from without; c, d, in section, with mycorhizic regions shaded; ^=antheridium 

 in longitudinal section ; /, ^=:archegonia. {a,d,c, d, x ■;; e,/, x 200.) lna,d, c,d, rt« = anthe- 

 ridium; a;- = archegonium ; £ or «;« = embryo. 



The Mycorhizic Type 



A third type of prothallus, differing widely from those described, is seen 

 in the Ophioglossaceae. It is as a rule colourless and develops beneath the 

 level of the soil in relation to mycorhizic nutrition. The wholly saprophytic 

 prothallus oi HelmintJiostacJiys investigated by Lang {Ann. of Bot. xvi, 1902, 

 p. 32) provides an example of these massive underground types, which offer 

 interesting analogies with those of the Lycopodiaceae and Psilotaceae. Each 

 prothallus consists of a lobed basal portion, or vegetative region, attached to 

 the soil by rhizoids. From this the cylindrical sexual region arises later, 

 usually growing vertically upwards, with a four-sided initial cell at the apex 

 (Fig. 274, a, b). But since the prothalli occur at a depth of about two inches 



