ORGANIC EVOLUTION 



131 



: porophyte early acquires a complete set 

 of foraging organs and becomes indepen- 

 dent of the parent gametophyte. The body 

 of the embryo grows out more slowly into 

 the underground horizontal stem (rhizome) 

 of the fern, producing as it grows new 

 leaves that rise to the light, and for a time 

 increase in size and complexity, and new 

 and larger roots that spread through the 

 soil. So, the sporophyte is launched upon 

 its career of independent existence; and 

 not being limited to the supply of food 

 that a small parent thallus can furnish, spore 

 production is long delayed. A long growth 

 Fig. 80. Diagram ii- period intervenes. A 



lustrating the intake , ., , ^ . 



of food materials at large plant body IS pro- 

 root and leaf in the ., ., 1 • 1 



auced, which when 

 mature develops spor- 

 angia in extraordinary 

 numbers upon the sur- 



fern, and the trans- 

 portation system of 

 vascular bundles 

 connecting the two 

 sources of supply 

 with all parts of the 

 plant body. 



^ face of its leaves. 



In this plant body the food absorbing 

 organs are those w4th which we have al- 

 ready become acquainted in the bryophytes. 

 The rhizoids surround the tips of the root- 

 lets in the soil (fig. 80). The assimilatory 

 parenchyma is chiefly located in the leaves, 

 protected by a layer of transparent epider- 

 mis, composed of thin flat, curiously inter- 

 locking cells (fig. 81). The oxygen of the 

 air finds ingress through pores (stomates) of the sort 

 already seen in the moss sporophyte (fig. 76 M, p). 



The development of a plant body of so greatly increased 

 size is made possible by the development of new structures 

 out of the parenchyma. These are of two sorts: — 



