HIDDEN MARRIAGES 



505 





FIG. 643. AN AKCHEGONIUM. 



An organ on the under side of the prothallium in 



which, after fertilization, the young fern-bud is 



produced. 



the contents by growth and division 

 rapidly develop into a minute green heart- 

 shaped scale (prothallium'), with a depres- 

 sion of the anterior margin, in which the 

 growing point is seated. The prothallium 

 as a whole consists of a single layer of 

 cells, but behind the growing point a 

 cushion of several layers is formed, from 

 one part of which root-hairs are produced. 

 On a portion of the cushion clear of root- 

 hairs the archegones will be found, and 

 among the root-hairs or on the margins 

 are the antherids. Cystopteris fragilis pro- 

 duces prothallia of two kinds, a smaller 

 bearing antherids only, and a larger bear- 

 ing both antherids and archegones. The 

 prothallia of Gymnogramme bear antherids 

 at first and archegones appear later (fig. 

 641). Each antherid contains compara- 

 tively few antherozoids, each being ribbon-shaped, coiled three or four 

 times, with a number of fine cilia at the fore end. The archegones have 

 a curved neck with a canal leading to the oosphere. The canal is formed 

 by the breaking up of a central row of cells whose walls and contents dis- 

 solve into mucilage, which is expelled and serves to retain the anthero- 

 zoids, which then make their way into the canal, some of them reaching 

 the oosphere and fertilizing it. Only one archegone on a prothallium is 

 fertilized ; and on many prothallia no fer- 

 tilization takes place. In some varieties 

 of Athyrium filix-fcemina and Aspidium 

 angulare prothallia bearing normal anthe- 

 rids and archegones are produced on the 

 fronds. The phenomenon is known as 

 apospory or suppression of the asexual 

 generation. Certain species of Asplenium 

 and Cystopteris produce buds on the fronds, 

 which grow into little plants without anv 

 fertilization. 



In Polypodiacese, which comprises 

 most of our native genera, the sporanges 

 have foot-stalks ; in all or nearly all the 

 other orders they are sessile. Each spor- 

 ange has originated in a single cell of the 



epiderm. It is a round, oval, or pear- FIG. 644. AN ANTHERIDIUM. 



shaped capsule, and when mature the walls Antherozoids escaping. 



