59 
which are circular towards the extremity, and 
hexagonal in the centre of the substance. In the 
first infancy of the vegetable, the pith occupies 
but a small space. It gradually dilates, and in 
annual shoots and young trees offers a consider- 
able diameter. In the more advanced age of the 
tree, acted on by the heart-wood, pressed by the 
new layers of the alburnum, it begins to diminish, 
and in very old forest trees becomes almost imper- 
ceptible. 
Many different opinions have prevailed with re- 
gard to the use of the pith. Dr. Hales supposed, 
that it was the great cause of the expansion and 
developement of the other parts of the plant ; 
that being the most interior, it was likewise the 
most acted upon of all the organs, and that from 
its re-action the phenomena of their developement 
and growth resulted. 
Linnaeus, whose lively imagination was continu- 
ally employed in endeavours to discover analogies 
between the animal and vegetable systems, con- 
ceived “ that the pith performed for the plant the 
same functions as the brain and nerves in animated 
beings.’" He considered it as the organ of irrita- 
bility, and the seat of life. 
The latest discoveries have proved, that these 
two opinions are equally erroneous. Mr. Knight 
has removed the pith in several young trees, and 
they continued to live and to increase. 
It is evidently then only an organ of secondary 
importance. In early shoots, in vigorous growth, 
it is filled with moisture, and it is a reservoir, 
perhaps, of fluid nourishment at the time it is 
