16^ 
ON THE DIFFUSION OF SEED. 
but when so great a number of them, and of their congeners of the class Syngenesia, 
are scattered about by the winds, it almost raises the chance to certainty that some 
of them will fall on spots where before there has been none, or only a scanty 
veg-etation ; on the tops of walls, for instance, where a thin stratum of soil has 
been formed by the decay of the winter crop of mosses. The process of the form- 
ing- of such soil is extremely interesting-, and may be observed, in a small scale, 
even in cities, on brick or stone walls. First, there is the green incrustation, called 
Byssus by Linnaeus ; but recently proved to be the primary germination of several 
mosses, such as Polytriclia and Tortula. When this decays, a very thin layer of 
vegetable earth is formed, which aifords a scanty support for the roots of the next 
year's crop of mosses ; and in process of time soil is formed of sufficient depth for 
Draba veriia and other wall plants. A singular contrivance is conspicuous in 
one of our wild cresses ( Cardamine impatiens), as well as in the balsams and in 
Touch-me-not {Impatiens noli-me-tangere), a native plant of the same genus. In 
all of these, when the seed is ripe, the valves which inclose it are so constructed 
that by the influence of the sun's heat they open with a sudden jerk, and throw 
the seeds to a considerable distance. The same effect is produced sooner and with 
more force when the ripe seed-vessel is touched by the hand, or by any accidental 
waving of the leaf against it. Were we disposed to refine upon the final cause of 
this, (a subject very ready to mislead,) we might say that this jerking of the seeds 
was contrived, not only for their diffusion, but for their preservation from birds and 
insects ; since the instant that these should begin to devour them, the springs of th'e 
valves would be thrown into action, and the seeds scattered about before a single 
one could be secured for a meal. In the wood sorrel (O.valis acetosella), as well as 
the horned sorrel (O. corniculata), the structure of these valves is very beautiful, 
but no description could do justice to it, not even with the aid of figures. The 
first, however, abounds in most woods ; and the latter, where it has been introduced 
as a flower, soon becomes, from the circumstance under consideration, a very 
troublesome weed. 
One of the most beautiful contrivances for the diffusion of seeds occurs in various 
species of violets. The seeds of this order of plants are contained in a capsule of a 
single loculament, consisting, however, of three valves. To the inner part of each 
of these valves the seeds are attached, and remain so for some time after the valves? 
in the process of ripening, have separated and stood open. The influence of the 
sun's keat, however, causes the sides of each valve to shrink and collapse, and in 
this state the edges press firmly upon the seed, which from being before apparently 
irregular in its arrangement, comes into a straight line. The seeds, it may be said 
are not only extremely smooth, polisher^, and shining, but regularly egg-shaped ; so 
that when pressed upon the collapsing edge of the valve, it slides gradually down 
the sloping parts of the seeds, and throws it with a jerk to a considerable distance. 
There is another part in the contrivance of Providence for the same purpose, in the 
Violacese, worthy of remark. Before the seed is ripe, the capsule hangs in a drooping 
position, with the persisting calyx spread over it hke an umbrella, to guard it 
from the rain and dews, which would retard the process of ripening ; but no sooner 
