CHAPTER VII. 



FERNS. 



1. Their germination. The spores of ferns usually 

 exhibit a tolerably thick, brittle, outer membrane, which is 

 furnished with prominent linear markings, or with wart- 

 like protuberances. When exposed to moisture and warmth 

 the inner membrane swells and ruptures the brittle outer 

 shell : this rupture usually occurs at the point of junction 

 of those three prominent lines of the outer membrane which 

 correspond with the lines of contact of the spore with the 

 three sister-spores which originated in the same mother- 

 cell and which, with the spore, formed a tetrahedron. In 

 the spores of those species in which the spore- mother-cell 

 divides into four cells having the form of quadrants of a 

 sphere and lying in one plane (which spores when ripe have 

 the shape of an elongated kidney) the exosporium usually 

 bursts by a longitudinal fissure, the course of which in like 

 manner corresponds with the line of contact of the spore 

 with its sister spores ; as for instance in Platycerium 

 alcicorne (PI. XXIV, fig. 1). A portion of the inner 

 membrane protrudes through the fissure of the exosporium, 

 and some chlorophyll-bodies are formed in this protruded 

 portion. The latter is soon afterwards separated by a par- 

 tition from that portion which remains inside the outer 

 membrane. In the outer of the two newly formed cells 

 the transverse division is repeated ; it usually occurs several 

 times (from three to five) in the terminal cell, so that the 

 young prothallium is converted into a row .of cells (PI. 

 XXIV, fig. 1). Sometimes the undermost cell, the one 

 which adjoins the exosporium, becomes considerably elon- 



