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THE INDIVIDUAL ORGANISM 



large size; and it contains a vascular system for transporting water and 

 other substances from one part of the plant to another. On account of the 

 latter characteristic, which is shared by the pteridophytes and the 

 spermatophytes, these two groups are often together called the vascular 

 plants. 



The pteridophytes include the ferns, club mosses, horsetails, and a few 

 other sorts of plants. Although, like the bryophytes, they are dependent 

 upon the presence of water for reproduction, in all other respects they are 



far more advanced and are closer 

 to the seed plants in type of in- 

 dividual organization. The roots of 

 the fern have root hairs, an apical 

 growing point and a root cap; 

 within there is a single central 

 vascular bundle of radial type. In 

 most ferns the stem is a horizontal, 

 subterranean rhizome which gives 

 off leaves from its upper surface 

 and roots from the underside. 1 

 Beneath the epidermis of the 

 rhizome there is commonly a 

 cylinder of mechanical tissue, en- 

 closing a mass of parenchyma cells 

 and from two to many vascular 

 bundles separated by a central 

 pith. Each vascular bundle is 

 composed of xylem more or less 

 completely surrounded by phloem, 

 the latter tissue sometimes forming 

 a continuous sheath outside the 

 xylem bundles. There is no cam- 

 is accomplished entirely by the 



Fig. 13.14. A fern, Polypodinmvirginianum, 

 to show growth habit. A, sporophyte, show- 

 ing rhizome, leaves with "fruit dots" (sori), 

 and roots. B, young sporophyte growing 

 from place where zygote was formed on 

 the mature gametophyte plant. {From 

 Hill, Over holts and Popp, Botany.) 



bium layer; growth of the stem 

 meristematic zone at its apex. 



True leaves are organs that appear for the first time in the pterido- 

 phytes. The leaves of ferns have a stalk and a more or less divided blade, 

 the latter often of large size. Unlike those of seed plants, the fern leaf 

 continues to grow for some time after it is formed ; when it emerges from 

 the ground, it is coiled like a watch spring, and it continues to elongate 

 at the tip while its older parts broaden and unroll. Another peculiarity 

 of the fern leaf is that each leaf vein, when it forks, divides into two ap- 



1 In the tree ferns the stem forms an erect trunk which attains a height of 50 feet 

 in a number of species and 80 feet in the tallest species known. The leaves of large tree 

 ferns may be as much as 20 feet long. 





