116 GROWTH OF PLANTS. 



sists of several concentric plates, united together by a 

 very fine layer of cellular tissue. 



M. Kieser takes a similar view, saying, that the sap 

 rises in the wood, and after having undergone in the 

 leaves a sort of respiration, it becomes pulp, in which 

 form it descends by the bark and is deposited between 

 the wood and the bark ; hence the formation of a new 

 layer of wood, and a new layer of bark. 



Professor Link, of Berlin, and more recently, M. 

 Dutrochet, support the opinion that the stem grows in 

 width as well as in thickness ; trees, according to this 

 view, being furnished with two systems, independent 

 of each other, each having a centre or vital organic 

 action, in opposite directions. The one system is cen- 

 tral, comprehending the pith, the hard wood, and the 

 pulp wood, the other is the tube of the bark, the 

 interior of which is composed of the inner bark. Each 

 of these systems acts on its own account, the result 

 being a simple extension of tissue, namely, a layer cf 

 pulp wood upon the pulp wood, and a layer of inner 

 bark upon the inner bark. 



M. Dutrcchet tried his first experiments upon a stem 

 of traveller's joy, the end of a young branch of which, 

 when cut across, he found to be composed of six bun- 

 dles of fibres running lengthways, and separated by wide 

 rayed spaces, in the centre of each of which spaces 

 new bundles of fibres are annually produced, so that 

 at the end of the second year, instead of six he found 

 thirty bundles, separated by an equal number of spaces. 

 This process ceases as soon as the wood is solid, but 

 always continues in the bark, while the roots show it 



