THE HIGHER CRYPTOGAMIA. 393 



the endosperm of the Coniferae — which in so many- 

 respects resembles the prothallhim of Selaginella — it is 

 formed by the attachment of originally free cells to the 

 inner wall of the large spherical firm-walled cell (the spore 

 or embryo-sac) in which it originated. 



In the above state the large spores are discharged from 

 the ruptured macrosporangium. In S. hortensis and hel- 

 vetica their further development is preceded by a dormant 

 condition which lasts for several months. During the latter, 

 the walls of those cells of the prothallium which adjoin the 

 spherical inner cavity of the spore become thickened. Thick- 

 ening layers are formed on both their sides, leaving, in some 

 cells, wide circular pits free (PL LVIII, figs. 1, 2). By 

 taking longitudinal sections of these pits it is seen that the 

 thickening of the walls is most considerable on that surface 

 which is turned towards the interior of the spore. The 

 contents of this large spherical cell secrete cellulose and 

 cause a thickening not only of the septa which separate the 

 contents from the cells which have attached themselves to the 

 inner wall of the apex of the spore, but also of the free 

 portion of the inner surface of the primary spore-membrane, 

 so that the latter becomes a very compact glassy membrane 

 iim" thick. The contents of the large cavity of the spore 

 consist during this period of a mixture of albuminous and 

 oily matter. These phenomena are most remarkable in S. 

 hortensis, less so in S. helvetica. In S. Martensi and other 

 tropical species the large spores germinate a few weeks after 

 being sown, and the above thickenings of the walls are 

 hardly perceptible (PL LVII, fig. 19). 



When the further development of the prothallium com- 

 mences, its cells divide repeatedly by longitudinal septa 

 perpendicular to the outer surface, and by transverse septa 

 parallel to that surface. This cell-multiplication begins 

 in the middle point of the prothallium, proceeds from 

 thence towards the periphery, and ceases long before it 

 reaches the latter (PL LVII, fig. 6). Before the repeti- 

 tion of the division by transverse septa archegonia are 

 formed. 



The first archegonium appears exactly at the apex of the 

 prothallium ; those which are lower down are of later origin. 



