196 HOl'MlilSTEtt, u.N 



only of all except one* of the archegonia of the same prothal- 

 lium, but by far the greater number of prothallia are en- 

 tirely abortive, and produce no young plant, It is not too 

 much to say that hardly one prothallium in ten produces 

 a frond-bearing plant, Prothallia which are devoid of 

 embryos, do not, however, by any means die ; when they 

 are not deprived of the conditions essential to their vitality, 

 that is to say, moderate warmth, subdued light, and 

 abundant moisture, they continue to develope themselves 

 for several months. In the simplest case the side lobes of 

 the prothallium increase very considerably in size ; they 

 overlap one another to a great extent in front of the inden- 

 tation of the fore edge. The prothallium when growing 

 exuberantly becomes circular ; it attains a diameter which 

 is from four to six times larger than that of the prothallia 

 of the same species which have produced young plants. 

 This is the regular rule in abortive prothallia of Gymnog- 

 ramma c/tiysq/j/ij/lla and others. The cushion of the under- 

 surface grows at the same time in thickness and in length. 

 The latter growth is produced by repeated division of the 

 cells which adjoin the bottom of the slit of the fore edge, and 

 which division takes place by means of septa at right angles 

 to the upper surface of the prothallium. At the same 

 time a number (often a very great number) of archegonia 

 are usually produced upon the prominent cushion of fleshy 

 cellular tissue on the under side of the prothallium. These 

 archegonia, with the rarest exceptions, are all abortive, 

 probably in consequence of the fact that no more new 

 antheridia are produced upon the hinder, oldest portions, of 

 the same prothallium. The earliest of these archegonia 

 have the same form as those which are produced contem- 

 poraneously with the latest antheridia. The later ones, 

 however, either develope a rudimentary neck only, or no 

 neck at all. The large central cell and the canal filled with 

 mucilage which leads to it appear to be sunk into the under 

 side of the prothallium (PI. XXVIII, fig. 2). The obser- 



* With the rarest exceptions, such as the case of Ceratopteris thalictroides, 

 observed by Merckliu (' Beobachtungen au Prothallium der Tarriikr.' Peters- 

 burg, 1850) and Pferis aquilina and Aspidium filix-mas observed by myself. 



