FRUCTIFICATION OF FERNS 



677 



yellowish or brownish outer coat, the exospore, which is marked 

 \*>ry much in the manner of pollen-grains (fig. 565) with points, 

 streaks, ridges, or reticulations. When placed upon a damp 

 surface, and exposed to a sufficiency of light and warmth, the. 

 spore begins to germinate, the first indication of its vegetative 

 activity being a slight enlargement, which is manifested in the 

 rounding off of its angles. This is followed by the putting forth 

 of a tubular prolongation (fig. 520, B, a) of the internal cell- wall or 

 endospore through an aperture in the outer spore-coat ; and mois- 

 ture being absorbed through this, r> the cell becomes so distended as 

 to burst the external unyielding integument, and soon begins to 

 elongate itself in a direction opposite to that of the first rhizoid. A 

 production of new cells by subdivision then takes place from its grow- 



FIG. 520. Development of prothallium of Pteris serrulata : A, spore set free from 

 the sporange ; B, spore beginning to germinate, putting forth the tubular pro- 

 longation a, from the principal cell b ; C, first-formed linear series of cells ; D, pro- 

 thallium taking the form of a leaf-like expansion ; a, first, and 6, second rhizoid ; 

 c, d, the two lobes, and e, the indentation between them ; /, /, first-formed part of 

 the prothallium ; g, external coat of the original spore ; k, h\ antherids. 



ing extremity ; this at first proceeds in a single series, so as to form 

 a kind of confervoid filament (C) ; but the cell-division soon takes 

 place transversely as well as longitudinally, so that a flattened leaf- 

 like expansion (D) is produced, so closely resembling that of a young 

 Mtircharttia as to be readily mistaken for it. This expansion, which 

 is termed the prothallium, varies in its configuration in different 

 species, but its essential structure always remains the same. From 

 its under surface are developed, not merely the rhizoids (a, b), which 

 serve at the same time to fix it in the soil and to supply it with 

 moisture, but also the antherids and archegones, which constitute 

 the true representatives of the essential parts of the flower of higher 

 plants. Some of the .former may be distinguished at an early period 

 of the development of the prothallium (h, h) ; and at the time of its 

 complete evolution these bodies are seen in considerable numbers, 



