lO BIOLOGICAL LECTURES. 



through the gametophyte into the earth, so that the sporophyte 

 could obtain water and the mineral constituents directly from 

 the earth, we should then have the sporophyte an entirely 

 independent organism. 



This completely independent sporophyte is first encountered 

 in the ferns or Pteridophytes — "vascular cryptogams," as 

 they are sometimes called. The simple liverwort-like game- 

 tophyte of the fern, as we have already stated, bears sexual 

 organs of very much the same structure as those of the liver- 

 worts, and from the egg-cell within the archegonium is pro- 

 duced, as the result of fertilization, a sporophyte which in its 

 early history follows very closely the development of the 

 sporophyte of a liverwort (Fig. 4, c). As a result of the early 

 divisions, a nearly globular mass of cells is formed, but very 

 soon a difference is manifest. Instead of retaining the original 

 globular form, as in Riccia, or simply forming a cylindrical 

 body, as in Anthoceros, the young fern sporophyte very early 

 develops several growing points (Fig. 4, d), which soon cause 

 it to lose its original form ; and it is evident that each of the 

 growing points is the beginning of a special organ. One of 

 these shows itself to be the cotyledon or first leaf, another 

 becomes the apex of the stem, and a third develops into the 

 primary root of the young sporophyte. This external differen- 

 tiation of the sporophyte, which by virtue of the root becomes 

 a self-supporting plant, at once sharply separates the ferns 

 from even the highest mosses. This development of special 

 organs is also associated with far more perfect tissues than are 

 ever found in the mosses, the most noteworthy of these being 

 the characteristic vascular bundles. 



Of the primary organs, the leaf or cotyledon- — at least in 

 the true ferns — soon assumes a flattened form, and the green 

 tissue of which it is mainly composed indicates that its prin- 

 cipal function is that of carbon assimilation. 



The root soon breaks through the tissues of the gametophyte 

 and grows downward into the substratum, thus putting the 

 young sporophyte into direct communication with the supply of 

 water. As soon as the sporophyte became thus entirely self- 

 supporting, a new type of plant was evolved which was destined 



