170 



GEOLOGY. 



the amphigens, gymnogens, and acrogens was to build 

 up, as we may say, the soil of the earth, and of a part of 

 them to furnish the coal which has been stored up for 

 the use of man. They are geological in their agency 

 that is, earth-making. But the chief object of the endo- 

 gens has been to furnish food to animals that is, animal- 

 making. They are zoological in their agency. The name 

 endogens comes from endon, within, and gennao, and 

 expresses the fact that the growth is by addition in all 

 the parts of the plant equally. In the trunk or stem, for 

 example, there is in every addition a formation of new 

 fibres intermingled with those already present, and in old 

 trunks or stems the hardest part of the wood is toward 

 the surface, and the softest toward the centre. Endoge- 

 nous plants have no pith and no distinct bark. 



253. Exogens. This name comes from ex, out, and 

 gennao. It indicates an entirely different mode of growth 

 from that of the endogens. There is a pith and distinct 

 bark, and the addition is in rings outside of those which 

 are already formed, the hardest part being toward the 

 centre, and the softest toward the surface. The differ- 

 ence in the modes of growth in the endogens and exo- 

 gens is exhibited by sections of stems or trunks in Figs. 



89 and 90. In the 

 endogenous stem, 

 Fig. 89, you see 

 the holes occasion- 

 ed by the section 

 of the circulating 

 vessels, and be- 

 tween these is the 

 mass of woody or 

 cellular tissue which makes up the substance of the stem, 

 and forms, indeed, the walls of the vessels themselves. 

 But in the exogenous stem you see the pith, the rings 

 of wood, and the bark. The lines which radiate like the 

 spokes of a wheel from the pith to the bark are called 



Fig. 89. 



Fig. 90. 





