I.S 



LIFK ; OL TLINKS OF GKNEKAL BIOLOGY 



means is such regulation eftectcd? The modem answer to this 

 question is twofold. Regulation is partly due to certain hormones 

 (chemical "messengers") which arc produced in "ductless glands" 

 and distributed by the blood. Thus the hormones of the thyroid 

 gland, and those of the piluitar>' body, have, among other functions, 

 that of growth-control. Again, it has been shown that parts where 

 mrtabolism is most intense, e.g. the growing point of a stem, exert 

 a s%*'ay or dominance over the growth of other parts, as we shall 

 see more fully later. 



Another feature of growth is its jxTiodicity. All are famihar 

 with the rings of growth on the cut stem of a tree, which mark 

 its years, through the well-marked seasonal alternation of spring 



\. Section through an Ulolilh of a IJony Fish, showing the .succc.s.sive lines 



of (IrjiDsition. 

 H. One «»f thr IV.ius of the (".ill ( ovrr or Oix?rculum of the same I-ish, showing 



thr Mi((rss>vc lines of growth. 



anrl sumuK r wckkI. which are different in texture. This instance is 

 no except ir.n.il ( as<\ but a vivid ilhi.stration of the rhythmic perio- 

 dicity of hfr. Tile same is M'cn in the zoning of fish-scales and the 

 barring (»f birds' feathers, and in the familiar growth-lines on the 

 slu'Ils of the seashore. 



lamiharity is apt to dull our eyes to the marvel of growth — the 

 annual covering (»f the brown earth with verdure; the desert 

 blossoming as the rose; the spreading of the green veil over the 

 miles (.f WfHKlland; the bamboo rising .so quickly that one can 

 s«-e it grow; the Se(pioia or P,ig Tree continuing to increase in bulk 

 fr»r three thous^uul years; the coral-|X)lyps adding chalice to chalice 



