CHAPTER IV. 



GROWTH BY THE STEM. 



OaiGIN OP THE STEM. THE GEO-WINa POINT. — ^PEODTJCTIOlir OP WOOD, BAEK 



PITH, MEDITLIABT EATS. PEOPESTIES OF SAP-WOOD, HBAET-WOOD, 



IIBEE, EIIfD, ETC. — ^NATirEE AND OFFICE OF LEAF-BITDS. — ^EMBETO- 

 BTTDS. — ^BtJlBS. — CONVETANCE OF SAP, AND ITS NATHBE. 



As soon as the root is fully in action, which is shortly after 

 it has begun to lengthen, the vitality of the living point that 

 exists at the bottom of the seed-leaves is excited, and a stem 

 begins to be formed. At first the stem is a mere point of 

 living matter, often invisible to the eye, but sometimes partially 

 developed ; in which latter case it is called the plumule. But, 

 as soon as nutritive matter is conveyed into it by the nascent 

 root, all its parts receive an impulse, which forces them into a 

 growth upwards; what matter already exists is distended, 

 enlarged, and solidified ; new matter is rapidly generated in all 

 directions from the vital centre, and if it were not for the 

 current setting upwards from the root, it would possibly grow 

 into a spherical figure. Pressed upon, however, by the 

 surrounding earth, impelled upwards by the current of sap 

 ascending from the root, and attracted into the air by the 

 necessity of respiration, the young stem assumes a cylindrical 

 form, its sides having a tendency to solidify, and its point to 

 grow longer. This point, or plumule, or first leaf-bud, soon 

 attracts to itself the food which the root procures from the 

 earth, and a part of the nutritive matter which is stored up in 

 the seed-leaves. It feeds especially upon the latter until they 

 are exhausted, and by the time this happens it is clothed with 

 leaves which are themselves able to feed it after the seed-leaves 



