NUTRITIVE ORGANS OF PLANTS. 109 
membranes or cell-walls being absorbed ; and at the same time the mem- 
brane which encloses the united cells thickens, by deposits taking place in 
its interior. Thus vessels are formed, which are closed tubes tapering 
to both extremities, and a substance called vascular tissue,! which 
is less soft and succulent than cellular tissue, and often becomes 
very tough and hard. Vascular tissue is generally found wherever 
strength is required, as in stems, the ribs of leaves, and the shells of 
nuts. A most important kind of vascular tissue is woody tissue, which in 
shrubs and trees becomes compacted into wood, the vessels being filled 
up by the deposits which take place in them, so that great solidity is 
attained. Not only is the wood of many trees of great value to man, 
but woody tissue, which never becomes compacted into wood, is also of 
much importance, as affording the fibrous materials of which cordage 
and clothing are made. Another kind of vascular tissue consists of 
spiral vessels, which are tubes of extreme fineness, sometimes not 
more than one three-thousandth part of an inch in diameter, coiled 
up within the young shoots and leaf-stalks of plants. These spiral 
vessels may be pulled out to a great length. If a young shoot or leaf- 
stalk of a geranium be broken, and the parts gently pulled asunder, the 
spiral vessels will be seen, delicate as the threads of a cobweb. Their 
great number, however, and their closely compacted coils, add to the 
strength of the shoot or leaf-stalk, whilst they contribute also to the 
circulation of its sap. 
Organs.—The organs of which plants consist are divided into two classes 
—Nutritive Organs and Reproductive Organs; the former being those 
which are essential to the life of the individual plant, the latter those 
which have for their purpose the perpetuation of the race. The nutritive 
organs are the root, stem, and leaves; the reproductive organs are the 
flowers and all organs concerned in the production of seeds. 
Nutritive Organs of Plants. 
The Root.—The roof not only serves the purpose of drawing nourish- 
ment from the soil, but also that of fixing the plant in its place, and it 
has often been remarked with admiration that trees growing in exposed 
situations send out more numerous roots than those which grow. in 
sheltered places, as if to anchor themselves more firmly, and to secure 
themselves against storms. 
Roots are often branched, and the branching is repeated again and 
again, particularly in hard and woody roots, which are divided at last into 
fine fibres. Softer roots are often quite unbranched, as in the hyacinth. 
The extremities of roots consist of loose cells, forming a minute spongy 
1 From Latin vasculum, a little vessel. 
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