ON OUTGROWTHS AND APPENDAGES.
(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.