ORGANIC EVOLUTION 



135 



between the other more specialized parts. The distribution 

 of these tissues in the bundles is indicated in figure 83. 



The distribution of the bundles 

 themselves is such that every con- 

 siderable part of the plant body is put 

 in vascular communication with every 

 other part. The differentiation of 

 vessels follows closely every growing 

 point, down into root and rootlet, up 

 into leafstalk and blade and lobe, 

 through every vein and veinlet. Some 

 branchlet of a vessel ends not far 

 from every group of rhizoids in the soil, 

 not far away from every stomate in the 

 leaf. The venation of the leaf (fig. 

 163), is the map of the distribution 

 of the vessels therein. 



The new organs of the fern are root 

 and leaf , both of them, mere extensions 

 of the plant body, carrying out into 

 new and wider foraging ground the 

 original foraging organs, rhizoids and 

 chlorophyl-bearing cells. The moss 

 has -no better circulatory apparatus than a simple axial 

 bundle of slightly elongated parenchyma cells; it develops 

 no roots and can forage in the soil only the length of its 

 rhizoids. Clearly, the structures of the fern we have just 

 noted sufficiently account for the larger size to which the 

 Pteridophytes have attained. 



Spore formation is greatly delayed, but in the end it occurs 

 on a much larger scale by reason of the large plant body 

 built up and capable of nourishing spores; moreover, it 

 may be repeated by the same sporophyte year after year. 

 In the bracken fern, sporangia (fig. 84) are developed 



FIG. 84. Sporangium and 

 spores of a fern, a , side 

 view; b, rear view- n, 

 the annulus; r, its front 

 end, which lifts on dry- 

 ing and ruptures the 

 wall below; 5. a spore 



