PROTHALLIUM OF FERNS. 357 



medium, best glycerine, the sporangia slowly open before our 

 eyes. This is best done with Aspidiwn; Scolopendrium does not 

 serve so well. The annulus thus becomes ultimately strongly 

 concave. Then follows, with a jerk, an opposite movement, 

 which more or less completely closes the sporangium. The 

 entire phenomenon can, in lessened degree, be repeated once or 

 several times. Careful observation shows that during the de- 

 hiscence the outer walls of the annulus project strongly into 

 their respective cells. The closing movement corresponds with 

 the moment in which, within the cells of the annulus, and at the 

 maximum of loss of water, a gas-bubble is separated out in each 

 cell. If gas has not come out in every cell, the outward curvature 

 still continues in those in which this has not happened, which 

 occasions the secondary movements of dehiscence. If now the 

 glycerine is replaced with water, the air- bubble in each cell de- 

 creases in size, and ultimately disappears, being resorbed by the 

 cell-sap, while the sporangium almost completely closes. With 

 renewed addition of glycerine, the reverse phenomenon can again 

 be produced. 



Tlie Prothallium. We select the Ferns, likewise, for the 

 purpose of studying the structure of the sexual organs in the 

 group of Vascular Cryptogams, and of following the processes of 

 fertilisation. The prothallium, which is the first and sexual 

 generation of Ferns, is always easy to procure. To find prothallia 

 in the open air is attended \vith considerable difficulty, and we 

 shall therefore do well to look for them in plant-houses, where on 

 damp shaded walls, on the stems of tree-ferns, or on flower-pots, 

 we can almost always find them. On the fibrous peat, much used 

 now in the culture of Orchids, Sarracenia, etc., and which is 

 often permeated by Poly podium vulgar e, are usually found numer- 

 ous prothallia of this fern, which we will here select for closer 

 study. As in most other Polypodiaceae, the prothallia of the 

 common Polypody fern have the form of small, heart-shaped, 

 bright-green leaves, lying on the substratum. We seize a pro- 

 thallium of medium size with the forceps, taking hold of the 

 place where it is attached to the substratum, and lift it away. 

 We immerse it in water, in which we move it for some time 

 to and fro, in order to wash off the fragments of adhering soil, 

 and then lay it, with the ventral (under) side upwards, in a drop 

 of water on the object-slide, and examine it under a cover-glass. 

 The prothallium, as already noted, is heart-shaped. It consists 



