6 



GENEKAli PRINOIPLES. 



species, as the apple, pear, quince, &c., small. In young 

 slioots it is soft, green, and succulent, and fills an impor- 

 tant part in their development. In the old part it is dry, 

 shrivelled, and seems incapable of taking any part in the 

 process of vegetation, and this appears evident from the 

 fact that ti'ees often continue to flom-ish after the centre, 

 containing the pith, has begun to decay. 



Sti'ucture of the Stem. — ^The stem is composed of 

 woody fibre and cellular tissue, a substance similar to the 

 pith. The woody fibre is arranged in perpendicular layers, 

 and the cellular tissue in horizontal layers, running from 

 the pith to the bark and connecting them. The mingling 

 of these two systems gives to the smface of the cross sec- 

 tion of a stem the beautiful veined or netted apjoearance 

 observable in fig. 2. The perpendicular layers of woody 

 fibre are most clearly observable when we cut a stem ver- 

 tically ; they are then easily sej^arated from one another. 

 The layers or plates of tissue radiating from the centre 

 to the stem are usually called tho medullary rays. 



The inner bark or liber, as has been stated, is, like the 

 wood, composed of thin layers of delicate perpendicular 

 fibres mixed with tissue. 



GrowtJh of the Stem. — The stem of a tree is originally 

 the extension of the cellular tissue of the seed. As soon as 

 leaves are formed thev organize new matter, which de- 

 scends and forms woody fibres: the layers sent down from 

 the first leaves are covered with those sent down from the 

 next, and so on, one layer after another is produced until 

 the end of the season, when the leaves fall and growth 

 ceases. A yearling tree has, therefore, a greater number 

 of layers of woody fibre at the collar than at the top, and 

 is, consequently, thicker; the second. }^ar the buds on 

 the '"r-t vt;j *s i:rov. tli i^roduce shoots, and these organize 

 new layers of woody libre, that descend and cover those 

 of tlip ];revions year, and thus growth proceeds from 



