ORGANIC EVOLUTION 



sporophyte 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 r 

 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. so. Diagram ii- period intervenes. A 



lustrating the intake , , 



of food materials at large plant body is pro- 

 root and leaf in the , 1 . . , 

 fern, and the trans- dUCed, which when 

 portation system of , . 



vascular " bundles mature develops spor- 



connecting the two . 



sources of supply angia in extraordinary 



with all parts of the , , 



plant body. numbers upon the sur- 



face of its leaves. 



In this plant body the food absorbing 

 organs are those with 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:- 



