(_Part of a Museum Lecture._)
Let us draw a clear distinction between “Appendages” and “Outgrowths.”
The prickles which are formed on the branch of a rose are appendages;
they may be detached without really breaking any part of the bush.
The spines which grow on the blackthorn are outgrowths, and cannot be
so detached. I wish that some better word than “appendages” could be
found, for it seems almost to imply insignificance, and many of the
appendages to plants are of the utmost importance. Still, it is true of
them all that they may be removed and yet leave the plant, as a plant,
complete, and many or most of them have only a transitory life, which
does not by any means equal that of the plant itself. They are like the
luncheon basket at the summer day’s ramble, not absolutely essential,
but very conducive to perfection. Now most forms of leaf, flower, fruit
and prickle are in this sense merely appendages. From the very earliest
stages of their formation arrangements exist for their separation, in
whole or part, from the plants on which they are produced. You will
see that I am cautious in my terms, and say in part or in whole, for
in truth some appendages never are detached as wholes, and very great
variety exists in the ways in which they are dealt with. For the most
part they are susceptible of death, and have their fixed duration
of life quite independently of the plant which bears them. In many
this independent death is the cause of their being cast off. In some
instances, however, it is not death, nor even sickness, but the fulness
of life and the attainment of adult age which causes them to leave the
parental home. I hold in my hand an oak twig with two empty acorn cups.
The acorns having attained maturity, have fallen out. Shall we say they
have detached themselves, or that the tree has detached them? They
have not fallen by mere weight, for they were doubtless nearly, if not
quite, as heavy whilst still green, and they were then firmly fixed.
You see at the bottom of the empty cup the large round scar which marks
the site of former attachment. It is brown and dry. It was by changes
which took place here that the acorn was loosened. The acorn had
ripened and ceased its growth. It no longer attracted sap through its
base of attachment, and the latter consequently became dry and brittle.
Possibly its feeding tubes were choked; at any rate, it is certain that
it underwent a sort of death and was no longer able to keep the acorn
in place. The process was much like that which occurs in the shedding
of leaves, with, however, the very noteworthy difference that the acorn
itself was still alive.
We have not, however, done with our oak twig. The acorns which it
bore were only appendages to an appendage, and it now becomes the
turn of the cups themselves and the whole of the long foot-stalk on
which they are mounted to become detached. These are no part of the
tree, and are of no use to it. They were developed in order to bear
flowers and fruit; that function they have now discharged, and they
must die. Life is preserved only by the discharge of function, or at
any rate the effort to discharge it. Utter inactivity leads to death,
and death leads to separation from the living and to decay. You see
that the whole foot-stalk is brown and shrunken and evidently dead.
This condition ends abruptly where the foot-stalk joins the stem. At
this spot, if you look carefully, you will see that there is a ring
of constriction, marking definitely where detachment is in progress.
This was the spot at which the production of the whole appendage began,
and here a sort of joint was left at which the final detachment was
destined to occur. Just one word of caution, that we must not carry
our distinctions too far. After all, they are to some extent matters
of degree. The joint which separates the appendage from the twig on
which it is produced can hardly be termed a true joint, for certain
structures run in unbroken continuity from the stem to the appendage.
These are the fibro-vascular bundles by which the appendage is fed and
also fixed in place. These bundles are usually quite visible in the
scar-surface left when a leaf or fruit is broken off. They are “the
nails in the horse-shoe” of the leaf-scar of the horse chestnut. Still,
it is certain that a sort of joint is present, and that the structures
are continuous in a very different sense from that of a stem or true
branch. You may observe this difference in my acorn-bearing twig, for
there are two acorn cups, and one has been produced by a branching out
from the stem of the other. This little branch is smoothly continuous
with the parent branch, and shows no preparation for detachment
whatever.
Thus we have seen that the arrangements under which leaves are shed
are exactly repeated in the case of fruits, and that it is by no means
needful that the object to be detached should be dead or dying. It
may perhaps surprise you to be told that sometimes appendages are
shed which have by no means accomplished their prospective work. Some
plants shed their flowers and do this deliberately, having made their
arrangements for a step which is apparently suicidal. In reality it is
not suicidal, nor is it one of limitation of population, but simply of
preferential employment of capital. The potato gives a good example of
this. Every spring you may see on the heads of this plant beautiful
flowers produced, which are destined in the course of another week to
be only flowerless foot-stalks. The flowers break off at a pre-existing
joint, just as leaves are shed. The influence which causes them to fall
is inability to attract sap, in consequence of inability to proceed to
the further stage of producing fruit. The young tubers underground make
such overpowering demands upon the sap-furnishing capabilities of the
roots that the flowers cannot obtain sufficient for their seed forming.
Thus they at once die: if not obviously, at any rate practically, and
detachment follows as a natural result.
It is a case of competitive growth and the tubers win. After a time the
plant will in the course of inheritance learn that it is useless to
produce flowers, will give up the attempt; indeed, many varieties have
already done so to a considerable extent.
No better instance could perhaps be given of the law which goes through
all animated Nature that activity is almost essential to continuance of
life, whether in individuals or their parts.
* * * * *
THE OLDEST FOSSILS (Lingula).--It is a noteworthy fact that these, the
oldest fossil animals known, belong to species by no means low in the
scale. Yet every trace of the many millions which must have preceded
them, and have gradually led up to their development, have perished.
The period of time which must have elapsed subsequent to the advent of
life upon the planet and the development of the lingula mollusc was
probably quite as long as that which has passed since the lingula left
its shell in the mud of the Portmadoc slate. The oldest fossils which
are known are found in the lower Cambrian rocks. They are small oval
shells, which were, during life, the protection of small soft-bodied
sea animals of highly complex structure: they had red blood. Their
descendants are still found in great numbers burrowing in sand on the
shores of tropical oceans. They have received the name of Lingula, and
have in turn conferred that name on certain hard rocks in which their
shells occur in abundance, the “Lingula flags” of Wales (RAY LANKESTER).
* * * * *
HUGE SHARK’S TEETH.--Ray Lankester figures, in his interesting lectures
on extinct animals from which we have quoted the above a gigantic
shark’s tooth. It is that of the _Carharodon megalodon_, and is three
times the length of the tooth of any living shark. Specimens of this
fossil tooth of smaller dimensions are common, and one should be found
in every museum. They are obtained from the bone bed of the Red Crag
at Felixstowe, but were not originally deposited in it. Many of them
have fragments of a yet older sandstone adhering to them. Lankester
calculates that his shark was 100 feet long.
* * * * *