PHYTOGENY. 213 



1. TRACHEAL-SYSTEM, WOOD. 



1077. With increased influence of light the tracheal 

 fasciculi also increase, and form a circle of columns in 

 the parenchyma around the centre of the plant. Between 

 the column, externally and internally to the same, is the 

 parenchyma. The more, however, the columns accumu- 

 late, by so much the more does it diminish, but before 

 the columns have stood singly in the parenchyma, it has 

 then the appearance as if narrow plates only of paren- 

 chyma traversed between the columns from without in- 

 wards. Finally the columns predominate to such an 

 extent and approximate so closely, that t\iQ plates almost 

 disappear. They are now called insertions of cellular 

 tissue, or medullary rays. As the tracheae convey air, 

 and have thus been more exposed to the process of 

 oxydation, they generally harden sooner than in other 

 parts. 



1078. Around the fasciculi of spiral vessels the cellu- 

 lar tissue also strives to elongate, and begins at the same 

 time to harden. Such extended cells, in which the light 

 has almost disappeared, are called fibres. Indurated 

 tracheae and fibres are called Wood. The wood is always 

 in the vicinity of the spiral vessels. It is a production 

 synchronous with the latter. 



1070. Only, where spiral vessels are, can genuine wood 

 originate ; but it is not everywhere, where they are 

 found, that woody fibres must be also present, although 

 the cells extend around all bundles of spiral vessels. If 

 the degree of oxydation of the cells be slight, they do 

 not harden, but continue herbaceous in texture. The 

 parenchyma has now been separated by a circle of fibrous 

 columns into an external and internal, or peripheric and 

 central. The central parenchyma becomes void of sap 

 and spongy, because the plant imbibing its nutriment 

 on the surface, and the air and the light operating 

 thereupon, the processes conduct it thither. This 

 withered parenchyma is called pith, which in accordance 

 with its origin merits no physiological consideration, 



