ORGANIC EVOLUTION 



133 



are easily recognizable by the finely perforate appearance 

 that the cut end of each bundle presents. These bundles, 

 like the internal strands of supporting tissue, may be traced 

 through the soft parenchyma by dissection, and followed 

 at their branchings out into the base of roots and leaves. 



Vascular Bundles. These bundles constitute the trans- 

 portation system of the sporophyte, condition its growth, 

 enable it to take possession of larger areas and deeper layers 

 of soil, to rise higher and to 

 spread out more widely in the 

 air and light, and are therefore, 

 structures of first importance 

 in the fern. They are com- 

 pound structures formed out of 

 the common undifferentiated 

 tissue (called the meristem) 

 by the ordinary processes of 

 cell growth and differentia- 

 tion. They are made up of a 

 variety of tissues serving vari- 

 ous purposes. The most 'im- 

 portant conducting tissues are two: i)tracheids, the tubes of 

 the largest diameter which give the perforate appearance to 

 the cross section of the bundle. These are the lignified 

 walls of elongated and empty cells, and serve chiefly for the 

 conduction of water, with whatever may be dissolved in it. 

 2) sieve tubes: these are living, greatly elongated, exten- 

 sively vacuolated, but yet protoplasmic cells, with oblique 

 overlapping ends whose walls exhibit groups of fine 

 perforations. Through the latter there is protoplasmic 

 continuity between adjacent cells. Albuminoid substances 

 are distributed through these cells by diffusion through the 

 continuous protoplasm. The chief supporting tissues of 

 the bundles are two: i) the tracheids already mentioned, 



FIG. 82. Diagram of a cross section 

 of the underground stem(rhizome) 

 of the fern (Pteris). e, epidermis; 

 /, vascular bundle; g, inner strands 

 of supporting tissue (sclerenchyma) ; 

 h, peripheral layer of supporting 



