6 VEGETABLE PHYSIOLOGY. 



of a wound on the stem of a tree healing over more 

 rapidly than the other sides; by the under sides of 

 horizontal stems or branches being farther extended 

 from the pith than the upper ; and by any deformity 

 on a stem increasing downward faster than in any 

 other direction. 



The pellicles of the cells are of various consistence 

 and properties as to durability. In the lower orders 

 of cellular es> as Fungi* for instance, they are muci- 

 laginous and fugitive ; in the higher orders of vascu- 

 lares they become ligneous ; and, along with the 

 secreted juices of the plant, constitute what is called 

 timber. 



Such plants as are wholly formed of cellular 

 matter, gain magnitude by a uniform swelling 

 motion from the centre outwards, Fig. 5. This 

 Fig. 5. 



progressive growth is continued for a longer or 

 shorter time according as the exciting circum- 

 stances are more or less favourable. A Boletus f 

 may be increased from a mere speck to a disk of two 

 feet in diameter ; and the common mushroom varies 



* Fungus. t The tree fungus. 



