CHAPTEE IX. 



THE BIOLOGY OF A PLANT {Continued). 



Reproduction and Development of the Brake or Fern, 



Reproduction. Unlike tlie earthworm, the fern reproduces 

 both by gamogenesis (sexually) and agamogefiesis (asexually). 

 Pteris possesses two modes of asexual reproduction, viz., the 

 detachment of entire branches from the rhizome and the con- 

 sequent establishment of independent plants, as already men- 

 tioned (p. Ill), and the formation of " adventitious buds " from 

 the bases of the leaf- stalks (Fig. 46). But besides these the 

 fern has a quite different method of reproduction, in which a 

 process of agamogenesis regularly alternates with gamogenesis 

 {alternation of generations). The folio whig brief outhne of 

 this important process may help to guide the student through 

 the subsequent detailed descriptions. 



Upon some of the leaves are formed organs called sijorangia 

 (Figs. 57, 63, 64), which produce numerous reproductive cells 

 called spores. The spores become detached from the parent and 

 develop into indejDendent plants, the prothallia (Fig. 70), w^hich 

 differ entirely in appearance from the fern and ultimately pro- 

 duce male and female germ-cells. The female cell of the pro- 

 thallium, if fertilized by a male cell, develops into an ordinary 

 ' ' fern, ' ' which again produces spores asexually. The forma- 

 tion and development of the spores is evidently a process of 

 agamogenesis., and the fern proper is therefore neither male nor 

 female — i.e., it is sexless or asexual. The formation and de- 

 velopment of the germ-cells, on the contrary, is a process of 

 gamogenesis j and the prothallium is a distinct sexual plant, 

 being both male and female (Jiermajyhi'odite or hisexual). In 

 general terms this is expressed by calling the ordinary fern the 

 spore-bearer, or sporophore^ and the prothallium the egg- 

 bearer, or oopJiore. The life -history of the fern, broadly 



130 



