An introduction to physiological and systematical botany/Chapter 7
CHAPTER VII.
OF THE MEDULLA OR PITH.
The centre or heart of the vegetable body, within the wood, contains the Medulla or Pith. This, in parts most endued with life, as roots, and young growing stems or branches, is a tolerably firm juicy substance, of an uniform texture, and commonly a pale green or yellowish colour. Such is its appearance in the young shoots of Elder in the spring; but in the very same branches, fully grown, the pith becomes dry, snow-white, highly cellular, and extremely light, capable of being compressed to almost nothing. So it appears likewise in the common Red or White Currant, and numerous other plants. In many annual stems the pith, abundant and very juicy while they are growing, becomes little more than a web, lining the hollow of the complete stem, as in some Thistles. Many grasses and umbelliferous plants, as Conium maculatum or Hemlock, have always hollow stems, lined only with a thin smooth coating of pith, exquisitely delicate and brilliant in its appearance.
Concerning the nature and functions of this part various opinions have been held.
Du Hamel considered it as merely cellular substance, connected with what is diffused through the whole plant, combining its various parts, but not performing any remarkable office in the vegetable œconomy.
Linnæus, on the contrary, thought it the seat of life and source of vegetation; that its vigour was the main cause of the propulsion of the branches, and that the seeds were more especially formed from it. This latter hypothesis is not better founded than his idea, already mentioned, of the pith adding new layers internally to the wood. In fact the pith is soon obliterated in the trunks of many trees, which nevertheless keep increasing, for a long series of years, by layers of wood added every year from the bark, even after the heart of the tree is become hollow from decay.
Some considerations have led me to hold a medium opinion between these two extremes. There is, in certain respects, an analogy between the medulla of plants and the nervous system of animals. It is no less assiduously protected than the spinal marrow or principal nerve. It is branched off and diffused through the plant, as nerves are through the animal. Hence it is not absurd to presume that it may, in like manner, give life and vigour to the whole, though by no means, any more than nerves, the organ or source of nourishment. It is certainly most vigorous and abundant in young and growing branches, and must be supposed to be subservient, in some way or other, to their increase. Mr. Lindsay of Jamaica, in a paper read long ago to the Royal Society, but not published, thought he demonstrated the medulla in the leaf-stalk of the Mimosa pudica, or Sensitive Plant, to be the seat of irritability, nor can I see any thing to invalidate this opinion.
Mr. Knight, in the Philosophical Transactions for 1801, p. 348, supposes the medulla may be a reservoir of moisture, to supply the leaves whenever an excess of perspiration renders such assistance necessary, and he has actually traced a direct communication by vessels between it and the leaf. "Plants," says that ingenious writer, "seem to require some such reservoir; for their young leaves are excessively tender, and they perspire much, and cannot, like animals, fly to the shade and the brook."
This idea of Mr. Knight's may derive considerable support from the consideration of bulbous-rooted grasses. The Common Catstail, Phleum pratense, Engl. Bot. t. 1076, when growing in pastures that are uniformly moist, has a fibrous root, but in dry situations, or such as are only occasionally wet, it acquires a bulbous one, whose inner substance is moist and fleshy, like the pith of young branches of trees. This is evidently a provision of Nature to guard the plant against too sudden a privation of moisture from the soil.
But, on the other hand, all the moisture in the medulla of a whole branch is, in some cases, too little to supply one hour's perspiration of a single leaf. Neither can I find that the moisture of the medulla varies, let the leaves be ever so flaccid. I cannot but incline therefore to the opinion that the medulla is rather a reservoir of vital energy, even in these bulbous grasses.
Mr. Knight has shown that the part in question may be removed without any great injury to a branch, or at least without immediate injury, but I have had no opportunity of making any experiments on this particular subject.