INTRODUCTION 



filamentous, and arises directly from the germinating spore ; 

 and upon the protonema, as a special branch or bud, the much 

 more complex gametophore is borne. Often, however, as in 

 many thallose Liverworts and Pteridophytes, the protonema is 

 not clearly distinguishable from the gametophore, or may be 

 completely sujjpressed. In the Pteridophytes the gametophyte 

 is, as a rule, much simpler than in the Bryophytes, resembling 

 most nearly the less specialised forms of the latter. In the so- 

 called heterosporous Pteridophytes the gametophyte becomes 

 extremely reduced and the vegetative part almost entirely 

 suppressed, and its whole cycle of development may, in 

 extreme cases, be completed within twenty-four hours or even 

 less. 



The non-sexual generation, or " sporophyte," arises normally 

 from the fertilised Q'g^-, but may in exceptional cases develop as 

 a bud from the gametophyte. In its simplest form all the 

 cells of the sporophyte, except a single layer upon the out- 

 side, give rise to spores, but in all the others there is developed 

 a certain amount of vegetative tissue as well, and the sporo- 

 phyte becomes to a limited extent self-supporting. In the 

 higher Bryophytes the sporophyte sometimes exceeds in size 

 the gametophyte, and develops an elaborate assimilative system 

 of tissues, abundantly supplied with chlorophyll and having an 

 epidermis with perfect stomata ; but even the most complex 

 moss-sporogonium is to a certain extent dependent upon the 

 gametophyte, with which it remains in close connection by 

 means of a special absorbent organ, the foot. In these highly 

 developed sporogonia the sporogenous tissue occupies but a 

 small space, by far the greater part of the tissue being purely 

 vegetative. 



In the Pteridophytes a great advance is made in the 

 sporophyte beyond the most complex forms found among the 

 Bryophytes. This advance is twofold, and consists both in an 

 external differentiation and a more perfect development of the 

 tissues. The earliest divisions of the embryo resemble very 

 closely those of the Bryophyte sporogonium, but at an early 

 stage four distinct organs are usually plainly distinguishable, 

 viz. stem, leaf, root, and foot. The last corresponds in some 

 degree to the same organ in the moss-sporogonium,* and like it 

 serves as an absorbent organ by which the young sporophyte 

 is supplied with nourishment from the gametophyte. In short. 



