i INTRODUCTION 5 



completely separated, and each one of these so-called "spore 

 mother cells," by division into four daughter cells, forms the 

 spores. The young spores are thin walled, but later the wall 

 becomes thicker and shows a division into two parts, one inner 

 layer, which generally shows the cellulose reaction and is called 

 the endospore (intine), and an outer more or less cuticularised 

 coat, the exospore (exine). In addition a third outer coat 

 (perinium, epispore) is very generally present. As the spore 

 ripens there is developed within it reserve food materials in 

 the form of starch, oil, and albuminous matter, and quite 

 frequently chlorophyll is present in large quantity. Some 

 spores retain their vitality but a short time, those of most 

 species of Equisctum and Osmunda, for example, germinating 

 with difficulty if kept more than a few days after they are 

 shed, and very soon losing their power of germination com- 

 pletely. -On the other hand, some species of Marsilia have 

 spores so tenacious of life that they germinate perfectly after 

 being kept for several years. 



From the germinating spore arises the gametophyte bear- 

 ing the sexual organs. Both archegonia and antheridia may 

 be borne upon the same plant, or they may be upon separate 

 ones. From the fertilised egg within the archegonium is pro- 

 duced the sporophyte or non-sexual generation, and from the 

 spores which it produces arise the sexual individuals again, 

 thus completing the cycle of development. 



On comparing the lower Archegoniates with the higher 

 ones, it is at once evident that the advance in structure consists 

 mainly in the very much greater development of the sporophyte. 

 In the Bryophytes, as a class, the gametophyte is more impor- 

 tant than the sporophyte, the latter being, physiologically, 

 merely a spore-fruit, which in many forms, e. g., Sphagnum, is 

 of relatively rare occurrence. The gametophyte in such forms 

 is perennial, and the same plant may produce a large number of 

 sporogonia, and at long intervals. The sporophyte in such 

 forms is small and simple in structure, and its main function 

 is spore formation, as it has but little power of independent 

 growth. In the Pteridophytes, on the other hand, the gameto- 

 phyte (prothallium) rarely produces more than one sporophyte, 

 and as soon as this, by the formation of a root and leaf, becomes 

 self-supporting, the gametophyte dies. In short, the sole 



