PROTHALLI AND EMBRYO OF SELAGINELLA 319 



certain other cases, the spores are retained within the spor- 

 angium until after the fertilisation of the archegonia produced 

 in the resulting prothalli. Both kinds of sporangia dehisce by 

 a wide slit, the spores ordinarily maturing their prothalli on the 

 ground. 



The contents of the microspore divide to form a few-celled, 

 exceedingly reduced male prothallus, developing neither chloro- 

 plasts nor rhizoids, and consisting in the main of a small number 

 of spermatozoid mother-cells (Fig. 182, /). The resulting biciliate 

 spermatozoids (Fig. 182, d) are liberated, during wet weather, 

 by the rupture of the coats of the microspores. 



The female prothallus begins to develop within the megaspore 

 long before the latter is shed, and, like the male, exhibits con- 

 siderable reduction and usually remains colourless. The early 

 stages of its formation are characterised by repeated division of 

 the megaspore-nucleus, separating walls only arising much later. 

 When ready for fertilisation the prothallus consists of a small- 

 celled tissue, situated opposite the apex of the tetrahedral spore 

 and exposed by the rupture of its coats at this point (Fig. 182, 

 a and b, p.), while the bulk of the spore-contents, still enclosed 

 in the megaspore- wall, are occupied by large cells laden with 

 food-material (/). The few archegonia, which have very short, 

 scarcely projecting necks (Fig. 182, e), are embedded in the 

 small-celled apical region (Fig. 182, a, b, ar.). 



The fertilised egg divides transversely, the inner (lower) half 

 giving rise to the embryo proper. The outer (upper) half, forming 

 the so-called suspensor (Fig. 182, b and c, 5.), divides a few times 

 and elongates considerably, so that the developing embryo is 

 pushed down into the large-celled nutritive tissue (/.) below. 

 The suspensor is a device characteristic of the higher seed-plants, 

 but in other essential respects the embryology resembles that 

 of Ferns. Absorption of the stored food takes place by means 

 of a sucker or foot (Fig. 182, c,ft.), but the young plant acquires 

 independence at an early stage (Fig. 182, g). 



Regarded as a whole, the Pteridophyta exhibit a great 

 variety of vegetative structure, and considerable specialisation 

 in their reproductive processes. They offer a marked contrast 

 to the Bryophyta in the relative importance of the spore- 

 producing phase, which is an independent plant highly adapted 



