310 GENERAL BOTANY 



The adult fern plant, which was described above, is produced 

 entirely from the stem portion of the embryo sporophyte. The 

 relation of the adult plant to the embryo is diagrammatically 

 illustrated in Fig. 178, , 6, c, in which the embryonic stem is rep- 

 resented as having elongated into the horizontal leaf-and- 

 root-bearing stem of the adult fern. In the figure the primary 

 structures root, foot, and first leaf are unshaded, while the 

 perennial adult fern plant is left shaded. This perennial fern 

 plant, therefore, corresponds to the moss capsule in the life cycle 

 of the two organisms, since both originate from the zygote and 

 both have for a main function the production of asexual spores. 



Life history. The essential stages in the life history of ferns 

 are the same as those of the mosses, in that the zygote develops 

 in each case a sporophyte which alternates with a gametophyte in 

 the life history. The spore is also in each case the means by 

 which the mosses and ferns multiply and disseminate their off- 

 spring, and it furnishes, therefore, a necessary motile stage in the 

 life history of these stationary land plants. 



The great advance made by the fern over the moss is in the 

 fact that the sporophyte in the fern is a highly organized plant 

 with true roots, stem, and leaves, in which highly differentiated 

 tissue systems provide amply for food making, food conduction, 

 and food storage. The fern sporophyte is therefore an independ- 

 ent food-making and spore-producing plant, quite different from 

 the leafless and rootless parasitic sporophyte of the moss. The 

 gametophyte, on the contrary, has become greatly reduced in the 

 fern and has for its sole functions the production of gametes and 

 the temporary support of the embryo sporophyte (Fig. 178, A). 

 The gametophyte is therefore short-lived and usually dies at the 

 close of a single season's growth. We shall see that this reduction 

 of the gametophyte to a comparatively simple plant structure 

 marks the beginning of the almost complete disappearance of this 

 phase of the life history in the highest seed plants. Since the ferns 

 are the undoubted ancestors of the early seed plants of the coal 

 period, the facts of their life history are of the utmost interest 

 and importance to a proper understanding of the conditions 

 which we are to discuss later in those plants. 



