34 
GARDENING AS A SCIENCE. 
the essential characteristics of the class. These may be described in the following 
order : — 
1st. A central column of pith, which extends no lower than to that point of 
division which is styled the collar, already described : this pith, or central column 
of cellular pulp, important as it is as a source of nutriment to the earliest develop- 
ments, may also be the conduit of those electric currents which connect the vital 
agencies of the tree with the earth through the medium of the absorbent roots. 
2nd. The pith is surrounded by a sheath^ in which are numbers of those 
wonderful appendages, called spiral tubes, to which (as they are generally and 
indispensably present in the earliest and future young developments) we incline 
to ascribe the flexibility and rotatory mobile power possessed by trees and shrubs 
in a degree so extraordinary ; but of this more must be said hereafter. 
3rd. Exterior to the medullary sheath we find layers of cellular tissue 
arranged in the form of rays, diverging more or less, as the spokes of a wheel, but 
interposed with vascular or ligneous fibres which are also incorporated with them. 
4th. The rays diverge till they approach or touch the outer integument, called 
the bark ; whence, according to Mr. Knight, they converge towards the pith. 
The above characters are traceable in stems of one year's growth. 
During the future progress of exogenous stems, the pith does not increase ; on 
the contrary, its column appears to contract ; and now the horizontal, cellular, 
rayey masses, appear more closely connected with the newly -formed layer of liber ^ 
and rather more remote from the central pith. The woody system of the first 
year becomes more compact, while the newly-developed system of descending 
fibres, with its interposed cellular rays, assumes the character of the tender, sappy 
wood of the present year, called Alburnum, 
When a student inspects the wood-cuts and descriptive plates of botanical 
writers, he is apt to conclude that the order of development and growth, is regular 
as the wheels and cogs of a highly-finished piece of machinery. And it must be 
confessed, that in the many specimens of horizontal slices which were sold as 
" Custances Vegetable Cuttings" the arrangement of the rays and interposed 
vascular descending vessels is beautifully symmetrical. Yet, the anomalies of 
Nature are very great, and much perplexity might be occasioned during more 
extended investigation, were it taken for granted that strict uniformity of structure 
must prevail. The inquiring reader will obtain much light on this subject by 
referring to the article "Exogens" in the Penny Cyclopsedia," elucidated by 
many figures. 
Any attempt to explain the origin and functions of the various constituents, 
must be attended with extreme difiiculty. The rudiments of every part certainly 
exist, and may be partly traced in the microscopic investigation of the seed: and 
here we may remove one source of error, by observing that all exogenous stems do 
not originate in seeds with two Cotyledons ; for there are many exceptions, some 
seeds having four, six, or more seed-lobes, but in every instance it appears that 
