128 MAIDEN-HAIR FERN. 



tion may either open more widely or almost close the 

 orifice. 



The prothallium, which is developed from a spore 

 produced by the leaf, bears little resemblance to the 

 mature spore-bearing fern plant. In its flattened shape, 

 cellular structure and rhizoids it does, however, have a 

 striking resemblance to the thalloid stem of Mar- 

 chantia. 



There are thus two distinct stages in the life history 

 of the fern : one is known as the vegetative, asexual or 

 pteridoid stage, in which the plant consists of stem, 

 roots and leaves, and produces spores, and, strangely 

 enough, answers to the sporogonium of the moss ; 

 the other, known as the reproductive, sexual or thal- 

 loid stage, 22 in which the plant consists of a prothal- 

 lium, on which the reproductive organs are borne, and 

 corresponds to the leafy plant in the moss. 



These reproductive organs are quite like those of 

 Marchantia and Atrichum. The antheridia consist 

 originally of one cell, which is later cut up into a cen- 

 tral cell and several parietal ones. The contents of 

 the central cell are divided into a number of small 

 spherical cells in which are formed the antherozoids. 

 When these are mature the parietal cells absorb water 

 and burst the apical one, thus permitting the anther- 

 ozoids to escape. The body of the antherozoid accord- 

 ing to Strasburger 23 is to be regarded as the proto- 

 plasm of the nucleus of the sperm cell, and the cilia as 



22 Pteridoid and thalloid are terms introduced by Underwood, Our 

 native ferns and their allies, p. 35. 



23 Das botanische Practicum, p. 455 ; Sachs, Text book, 2nd Eng. 

 ed., p. 423. 



