6 GENERAL PKESTCIPLES. 



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

 shoots it is soft, green, and succulent, and fills an imj)or- 

 tant part in their develoj)ment. 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 trees often continue to flom-ish after the centre, 

 containing the pith, has begun to decay. 



Structure 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 sm-face of the cross sec- 

 tion of a stem the beautiful veined or netted appearance 

 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 separated from one another. 

 The layers or plates of tissue radiating from the centre 

 to the stem are usually called the medullary rays. 



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

 wood, composed of thin layers of delicate j)erpendicular 

 fibres mixed with tissue. 



Growth 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 they 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 year the buds on 

 the first year's growth produce shoots, and these organize 

 new layers of woody fibre, that descend and cover those 

 of the previous year, and thus growth proceeds from 



