ANATOMY OF PLANTS. 



189 



burnum {splint swacJie) proceeds from their bad situation, 

 and from other causes, which hinder the quiet lateral im- 

 pulse, (416.) 



As the growth of most trees takes place in determinate pe- 

 riods, it is from this circumstance that the annual rings, 

 which we observe in the wood, take their origin. The ear- 

 liest spring growth is commonly the richest ; hence the great- 

 est number of new layers are deposited by it, but, on ac- 

 count of the continued ascent of the sap during the summer, 

 these layers do not experience the gentle lateral pressure to 

 such a degree, as is necessary for the thickening of the sides 

 of the cells and of the sap-vessels, and for the consequent 

 production of wood. The second growth proceeds somewhat 

 more softly ; fewer new layers are deposited ; but the sub- 

 sequent lateral pressure from the bark-cells assists the thick- 

 ening and hardening of the wood more powerfully, on which 

 account the outermost layers of any annual circle are always 

 the firmest and the richest in resinous and oily juices. In 

 many tropical trees, the same distinction of the annual 

 rings is perceived, because in these also there is a periodical 

 change of vegetation during the dry and wet season. On the 

 other hand, the wood of many trees even of our climate shews 

 no annual circles, because they either undergo no double pres- 

 sure from the sap, or because their organisation resists the 

 alternate thickening we have mentioned. Even what is call- ^ 

 ed the Silver-grain {Quer-gefuge), is not equally distinct 

 in all woods, — although it is present in them all, since in all 

 cases the bark pushes the silvery horizontal processes towards 

 the pith. The firmest woods have commonly the most dis- 

 tinct silver-grain, as may be seen in the Oak, the Beech, and 

 the Elm. 



299. 



The innermost part of the stem, namely the Pith, is as 

 completely cellular as the bark. In young shoots it is full 

 of sap, and is closely united with the woody circles. At a 

 later period the juices are dissipated ; the pith becomes dry 

 and white, and seems no longer to be so closely connected 

 with the wood. The quicker the plant grows, the more is 



