INTRODUCTION. 



greater number of British genera, normally sixty-four irregular- 

 shaped, rough-coated microscopic bodies. These spores are of 

 uniform structure, and are simple cells. On the absorption of 

 moisture the contained protoplasm bursts through the double 

 walls at one point, and spreads into a minute green, heart- 

 shaped scale called a prothallium. The greater part of this 

 body consists of a single layer of cells, and the growing point 

 lies in the notch at the broader end of the heart-shape. In the 

 Moonwort (Botrychiutri), however, the prothallium is an under- 

 ground tuberous body, producing sexual organs on its upper 

 surface ; and in the Filmy Ferns (Trichomanes and Hymeno- 

 phyllunf) it is reduced to a branching thread. In the Brittle 

 Bladder Fern {Cystopteris fragilis) there are two forms of pro- 

 thallium a smaller producing male organs (antherids) only, 

 and a larger with female organs (archegones] and male organs. 

 In the Horsetails (Equisitince\ though the spores are all alike, 

 some produce male prothallia, others female prothallia. In the 

 Water-ferns (Hydropteridce) and Club-mosses (Lycopodince) 

 there is a similar sexual distinction of prothallia but a difference 

 in the spores, the male prothallia being produced by smaller 

 spores (microspores). 



These sexual organs are really the counterparts of the anthers 

 and ovaries of the flowering plants, and are of tolerably uniform 

 structure in all the species. With the exceptions noted in the 

 foregoing paragraph, they will be found on the underside of the 

 prothallium projecting from a thickened cushion just behind 

 the notch and growing point. The antherids are spherical 

 bodies with a lid, and contain each a small number of ribbon- 

 shaped antherozoids, which are provided in front with fine 

 filaments (cilia) of protoplasmic gossamer. These antherozoids 

 agree in value with the pollen-grains of flowering plants, though 



