THE ERIAN OR DEVONIAN FORESTS. 69 



mud of the black shales. We have thus a remarkable 

 example of a group of plants reduced in modern times to 

 a few insignificant forms, but which played a great role in 

 the ancient Palaeozoic world. 



Leaving the Khizocarps, we may now turn to certain 

 other families of Erian plants. The first to attract our 

 attention in this age would naturally be the Lycopods, 

 the club-mosses or ground-pines, which in Canada and 

 the Eastern States carpet the ground in many parts of 

 our woods, and are so available for the winter decoration 

 of our houses and public buildings. If we fancy one of 

 these humble but graceful plants enlarged to the dimen- 

 sions of a tree, we shall have an idea of a Lepidodendron, 

 or of any of its allies (Figs. 15, 21). These large lycopo- 

 diaceous trees, which in different specific and generic 

 forms were probably dominant in the Erian woods, re- 

 sembled in general those of modern times in their fruit 

 and foliage, except that their cones were large, and prob- 

 ably in most cases with two kinds of spores, and their 

 leaves were also often very long, thus bearing a due pro- 

 portion to the trees which they clothed. Their thick 

 stems required, however, more strength than is necessary 

 in their diminutive successors, and to meet this want 

 some remarkable structures were introduced similar to 

 those now found only in the stems of plants of higher 

 rank. The cells and vessels of all plants consist of thin 

 walls of woody matter, enclosing the sap and other con- 

 tents of these sacs and tubes, and when strength is re- 

 quired it is obtained by lining their interior with suc- 

 cessive coats of the hardest form of woody matter, usually 

 known as lignin. But while the walls remain thin, they 

 afford free passage to the sap to nourish every part. If 

 thickened all over, they would become impervious to sap, 

 and therefore unsuited to one of their most important 

 functions. These two ends of strength and permeability 

 are secured by partial linings of lignin, leaving portions of 

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