16 
the previous year, while, at the same time, a new fibrous layer is added to the 
inside of the bark. These layers are developed out of the vitally active cells of 
the Cambium layer, which is situated outside of the indefinite vascular bundles 
which form the wood. The medullary rays, at the same time, increase by addi- 
tion to their exterior ends, and thus continue to keep up their connection 
‘between the pith and the bark. In succeeding years we have, in like manner, 
new layers of wood and fibrous bark, one of each every year ; and the medullary 
vays also continue to grow from within outwards ; there are also other medullary 
rays, which connect each annual zone with the bark ; these are called secondary 
medullary rays. Each succeeding yearly growth is, therefore, a repetition of the 
first, except as regards the pith ; this never increases in size after the first year, 
The second object is the piece of straw. Not a few of you, I imagine, will be 
surprised to hear me call a stalk of wheat a tall, symmetrical, tower of stone sur- 
rounded by a casing of wood; such, however, with all propriety, it may be called. 
It will be at once seen that, to keep the grain from the moisture of the ground, 
in order that it may ripen properly, a tall stem must be developed ; and also in 
the case of most of the economic grain-bearing grasses, which produce such 
enormous crops in comparison to the size of the plants, that it must be of excep- 
tional strength to bear the weight of the copious ear. Nature has effected this 
by a most beautiful and unexpected process. The cuticle in some plants, but 
especially in these which we are considering, has its cell walls impregnated with 
silex, or flint; so much so, that when all the organic matter has been removed 
by heat or prolonged masceration in diluted nitric acid, the forms of the cuticle 
cells, hairs and stomata are distinctly marked out in silex, and can be beautifully 
displayed under the microscope, with the aid of polarized light. Such silicious 
euticles are also found in the husks of the grains yielded by these plants, and 
the hairs, with which the palee or chaff scales of most grasses are furnished, are 
strengthened by a like deposit. Moreover, the vegetable cells produced in straw 
are prosenchymatous and form fibrous-tissue, which, added to the silicious 
skeleton, and the shape of the stem, a hollow cylinder, give the great strength 
and flexibility necessary. A stem of wheat straw is, perhaps, one of the 
strongest structures, for the amount of substance in it, which can be found in 
nature. An ordinary thick stem would, of course, support the weight of the ear as 
well, but. as the seeds of these plants were to form the large portion of the food of 
the human race, which they do, it was necessary to provide them with stems which 
would take up as small an amount of space as possible. I have no doubt that, 
did the members of the grass tribes cover as much, area, in comparison to the 
amount of fruit they produce, as other plants, the whole world would not suffice 
cas a field to grow the amount annually required to sustain man. 
Our next object is the leaf. There is no part of a plant, not even the flower, 
with its wealth of beauty and delicious scent, which deserves more careful 
consideration, nor which performs more important functions than the foliage, 
the source of that great charm in the landscape, which all feel so keenly in the 
Spring, when nature has spread, with lavish hand, her mantle of refreshing green 
