VEGETABLE ORGANISATION. 
431 
which give the appearance of a tessellated or rectangular net-work to 
the vascular structure. In the acotyledons, the leaves or leafy expan¬ 
sions are destitute of ribs or veins, and consist merely of a pulpy cel¬ 
lular tissue, enveloped by a cuticular expansion. The development of 
these parts from the simple vesicle or cell may be thus supposed to 
arise :—Upon the internal surface of the envelope of a primary cell, 
two secondary vesicles may follow the growth of the generating or 
primary cell,, and stretch out longitudinally; they will thus form two 
lateral lobes, whose interstice will be the median nerve. If other 
vesicles be developed within these two cells, and become in their turn 
large tertiary cells, their interstices will form the lateral nerves, and 
will seem to derive their origin from the median nerve. Other vesicles 
may, in the same way, be developed into cells in each of the tertiary 
cells, and others again within these, and so on indefinitely; and a 
primary microscopic vesicle will be thus transformed, almost before our 
eyes, into a leaf of a dicotyledonous plant. At the same time that this 
development of the cellular texture of a leaf is going on, a vascular 
connexion with the sap-vessels of the branchlet to which it is attached, 
may be formed through the medium of the hilum, or point of attach¬ 
ment of the primary cell, and with this again throughout the leaf, by 
the hila of the secondary, tertiary, and succeeding orders of the cells, 
the interstitial spaces which form the nerves becoming vascular, in con ¬ 
sequence of the development of elongated cells or tubes through these 
points of attachment. 
“ The structure of the floral coverings is analogous to that of the 
leaves ; but the variety and elegance of their external forms, and the 
beauty of their colours, are such as to excite our deepest admiration. 
The stamens and pistils, or those parts which are more immediately 
concerned in the process of fructification, and for the preservation and 
due elaboration of which the more conspicuous paTts of the flower are 
destined, consist, apparently, of cellular texture only. 
“ The most important part of the fruit is the seed. This organ con¬ 
sists, at least in the flowering plants, of an embryo, or the rudiments 
of the future plant, and various envelopes, generally two, sometimes 
three or four. The essential parts of the embryo are, the plumula , or 
young plantlet—the radicle, or future root—and the cotyledons, or 
seed-lobes, destined for the nourishment of the other portions in the 
first stage of their growth. Another important part of the seed, though 
not discernible in all plants, is the albumen—that which forms the 
principal part of the seed in the Cereales and grasses, and which, in 
one of this tribe—the Triticum hybernum , or wheat—is of such vast 
and incalculable importance to the welfare of the human race. 
