30 
THE FLORIST'S JOURNAL, 
found in it. The medullary sheath consists of spiral vessels, im¬ 
mediately surrounding the pith, from which it diverges when¬ 
ever a leaf is formed; and thus it is in direct communication 
with the leaf-buds and the veins of the leaves, and establishes a 
connexion with the axis, and its appendages, which in all cases 
are prolongations of it. The ivood lies on the medullary sheath, 
and consists of annually deposited concentric layers, formed by 
the successive adhesion of the descending fibres of the buds, 
and by the interposition of the medullary rays, connecting the 
pith and the bark: these layers consist of woody and vasiform 
tissue, either in distinct strata, the woody tissue being outer¬ 
most, or intermingled together, and nearly obliterating the 
appearance of zones: when the tissue of these layers becomes 
filled with secretions it ceases to perform vital functions, and 
in this fully-formed state is called heart-wood; if in a living 
state, and imperfectly formed, it is called alburnum. The bark , 
or external coating of the stem, when but one year old, consists 
of a cellular integument, called the cuticle or epidermis , already 
noticed, and an internal lining of woody tissue, called the liber 
or inner bark: similar layers of cellular integuments and woody 
tissue are annually formed, internally with respect to the exist¬ 
ing bark, and therefore in contact with the wood. The medullary 
rays consist of thin plates of cellular matter connecting together 
the tissue of the trunk: the cellular matter in this form is 
termed muriform. Between the bark and the wood, in the 
spring, there is interposed a viscid matter, called cambium , 
which, when magnified, is observed to consist of transparent 
granules of cellular organisation, and which is supposed to be 
destined to afford a proper pabulum to the descending fibres of 
a bud; in all probability it is the organising matter in which 
the cellular tissue of the medullary sheath is engendered. 
The functions of the stem are to bear the leaves and other ap¬ 
pendages of the axis, and to expose them to the action of light 
and of the atmosphere; to convey fluids both from the root 
upwards, and from above downwards; and to store up the 
peculiar secretions of the plant. The pith has had various 
functions assigned to it, analagous to those of the brain, the 
lungs, the stomach, the nerves, and the spinal marrow, in 
animals: its real use is probably to serve as a receptacle for the 
sap during the infant stages of growth, by means of which the 
tender vessels surrounding it gain their nourishment. The 
