THE EVOLUTION OF THE SPOROPHYTE. n 



to become the predominant type of the future. The develop- 

 ment of a sporophyte with definite axis or stem, to which are 

 attached leaves and roots, is the characteristic of all the higher 

 plants, or the "vascular plants" as they are sometimes called. 



In the ferns, spore production, which in the lower Archego- 

 niates is the sole function of the sporophyte, becomes entirely 

 subordinated to the vegetative existence of the sporophyte, 

 which may reach a very large size, some of the tree ferns hav- 

 ing stems ten metres or more in height, and living for many 

 years. Spore formation in these forms does not take place 

 until several years after the sporophyte is first formed. Finally, 

 however, the spores are developed from a sporogenous tissue, 

 exactly as in the liverworts and mosses. The spores are, how- 

 ever, borne in special organs — sporangia, which in the ordinary 

 ferns are borne in groups on the back of the leaf. The form 

 and position of the sporangia are quite constant and form one 

 of the best means of classifying the Pteridophytes. The first 

 hint of a segregation of the sporogenous cells is met with in 

 Anthoceros and its allies, where the continuous layer of spo- 

 rogenous tissue is imperfectly divided into fertile and sterile por- 

 tions (Fig. 4, a). This sterilization of a part of the sporogenous 

 tissue is probably the first step in the direction of the special 

 spore-bearing organs, or sporangia, to which the spore formation 

 is restricted in the Pteridophytes. The development of simple 

 sporangia probably occurred very early in the history of the 

 group. 



With the growing importance of the sporophyte is associated 

 the development of very perfect tissues of different kinds, 

 most of which we already find present in the ferns. Of these, 

 the vascular bundles already referred to are the most charac- 

 teristic. It is not unlikely that the central strand of elongated 

 cells, occupying the axis of the sporophyte in Anthoceros and 

 in most of the true mosses, may be compared to a rudimentary 

 vascular bundle, but none of the Bryophytes show the charac- 

 teristic tracheary tissue, the woody part of the bundle in the 

 higher plants. 



In all the plants above the mosses it is the sporophyte which 

 attracts our attention. We cannot here go into details as to 



