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70 TYPES OF BRITISH PLANTS 
formed by the extremity of the bundles, which mass 
together to form the main part of the stem or trunk, 
as it may be, of the plant. Well, in moss leaves we find 
the system beginning, for one can notice at once in most 
of them a “mid-rib,” which is different from the general 
substance of the leaf. This is composed, not of vessels, 
but of a double or triple row of specially constructed 
cells, which do not seem to take part in the manufaetur- 
ing duties of the leaf, but foreshadow the duties of the 
bundles by acting chiefly as conductors of the products 
of the other cells in the leaf. This mid-rib is often 
continued right into the stem, and we see that it is not 
a very long step for them to combine into the central 
mass we discussed in Chapter III We also find that 
there is now quite a distinct skin, a layer of coleurless 
or darkened cells, which contain no chlorophyll, and 
which have for their duties simply the protection of the 
working cells within from heat, cold, and other injuries 
that might retard their action. Up the centre of the 
stem there runs also, not a vessel, but the next thing 
to it, a column of specially elongated cells, whose walls 
are strengthened so as to give support to the plant, whilst 
at the same time they are good conductors of nourish- 
ment from part to part. 
This is a general outline of the moss structure; and 
now we come to the question of their methods of per- - 
forming a plant’s great duty, the reproduction of its 
kind. There are the two methods, vegetative reproduc- 
tion and reproduction by seeds or spores. (Of course 
flowering plants are the only plants that can produce 
seeds. The rest, ferns, mosses, funguses, ete., have to 
be content with spores.) Mosses employ the first method 
to a vast extent, and, being rather low in organisation, 
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