30 THE BRITISH FERNS. 
according to Prof. Henfrey,* who has thoroughly investigated the 
subject, consists of a bursting of the tough outer coat of the spore, and 
a protrusion of the delicate inner membrane in the form of a little 
tubular pouch, the contents of which soon begin to acquire a green 
colour. This tubular process elongates, and at length becomes 
divided by cross partitions, a row of five or six cells being sometimes 
formed. The first formation of root fibres, which often occurs in 
the earliest stages, consists of the growth outwards of the wall of one 
or more of the cells of the filamentous portion of the prothallus into 
a long slender undivided tube, and the roots of the full grown pro- 
thallus exhibit the same characters, being tubular prolongations of the 
inferior walls of the cells of the green frondose expansion. After 
a time the youngest cell of the growing prothallus becomes more 
expanded in the transverse diameter, and after the next transverse 
division of the cavity, a new mode of increase occurs; the newest 
cell becomes divided in a direction parallel to the original direction 
of growth. It is by the repetition of these two modes of extension | 
that the prothallus gradually acquires its flat expanded form. When 
the prothallus has attained some size, the cells about the middle of the 
front border are produced smaller than those at the sides and anterior 
angles, so that the latter advance forwards as rounded lobes, leaving 
a sinus, or notch, in the centre, the prothallus thus acquiring a kind 
of inversely heart-shaped figure. Numerous radical hairs are pro- 
duced about the posterior part of the prothallus, and in the same 
part a process of horizontal cell division is commenced, so that a 
thickness of two, three, or more cells is formed. 
If a leafy stem is produced, the prothallus dies away; but if none 
of the pistillidia become fertilised, so that the prothallus remains 
- barren, its vegetative existence may be indefinitely prolonged. In 
this case the lateral lobes enlarge, and their margins become 
variously curved, sinuous, or convoluted; and new lobes sometimes 
* Henfrey, Transactions of the Linnean Society of London, xxi. 117. 

