346 PART IV. — VEGETABLE TAXONOMY. 



In the reproductive process, also, great progress is shown 

 over that which occurs in the Mosses. They exhibit, like the 

 Bryophyta, a distinct alternation of generations, but while in the 

 latter group the sexual plant is the more conspicuous and better 

 developed, the reverse is the case in the Pteridophyta, the plant 

 producing the sexual organs being, even in those species in 

 which it is best developed, a mere thallus resembling that of 

 some of the lower Hepaticse, and commonly perishing soon 

 after the fertilization of the germ-cell has been effected, while 

 the asexual plant developed from the latter, is conspicuous and 

 highly organized. 



The process of reproduction, briefly outlined, is as follows : 

 The spore borne by the asexual plant germinates and produces 

 the sexual plant, which since it is temporary in its character, a 

 mere forerunner, so to speak, of the plant which is ultimately 

 produced by it, is called a prothallium. 



In the Ferns, Equisetums and other species in which it is 

 most highly developed, it consists of a flattened body attaching 

 itself to moist soil by means of hairs emitted from the under 

 surface, and is always insignificant in size compared with the 

 plant which produced the spore. It is composed of chlorophyll- 

 bearing cells, and may continue its growth for some time before 

 bearing fructifying organs. These consist of antheridia and 

 archegonia. The archegonia, like those of the Mosses, are flask- 

 shaped, cellular structures, having an enlarged basal portion 

 which contains the germ-cell or oosphere, and a neck through 

 which the fertilizing cells penetrate. The basal portion is buried 

 in the tissues of the thallus, but the neck is free, projecting 

 above the surface. The latter is usually short and composed of 

 four longitudinal rows of cells. 



In the interior of the young archegonium, an axial row of 

 three cells is formed, the lower constituting the ottsphere, the 

 other two the neck cells, which later are converted into mucilage, 

 the pressure of which forces apart the cells composing the wall 

 of the neck, forming a passage for the antherozoids. The mucil- 

 age which oozes out of the opening seems to have an important 

 influence also in directing the antherozoids to their destination. 



The antheridia may be produced on another part of the same 

 prothallium, or on a different one. They are usually rounded 



