26 



KNOWLEDGE. 



[Febkuaby 1, 1901. 



by the end of autumn its period of life is fulfilled; 

 therefore a finn anchoring can be dispensed with. By 

 way of contrast,- consider the roots of a young tree. 

 This plant has a long life before it, and the future must 

 be cai'efully provided for. The roots are long and 

 very strong and woody — much tougher often than the 

 branches, for they axe subject to greater stress. They 

 spread widely, and hold the plant in its place with 

 wonderful tenacity — as we realize when we try to weed 

 out a few Ash or Sycamore seedlings that have got into 

 our flower-beds. In the case of biennial plants, which 

 during the first season of their existence form a rosette 

 of leaves on a very shortened stem, and in the following 

 yeai- shoot up. flower, and die, the root is often much 

 thickened, and u.sed for the storing up during the first 

 season of the plant-food required for the great vegetative 

 efi'ort of the second. Our lai-ger native thistles furnish 

 good examples of such foresight on the plant's part. 

 Gardeners have taken advantage of this tendency of 

 some species to enlarged roots to develop the character 

 by continued selection of the most fleshy-rooted, and 

 to their successful efforts we owe the juic}' caiTot, the 

 coi-jDulent tiu'nip and beet, tlie salsify and parsnip. 

 Biennial or perennial plants which dwell on ai-id plains, 

 or in sandy ground, have particular need of long and 

 fleshy roots, that they may store up food and water 

 against times of drought, and in dry weather be able 

 to tap the deeper damper layers of soil. If we examine 

 the plants which grow on the dunes or on the sea shore, 

 we shall find that many of them have a long fleshy 

 tap-root which fulfils these pui-jjoses — the Sea-spurge for 

 instance, and the lovely Horned Poppy. Note likewise 

 the great length and succulence of the roots of the Sea^ 

 Holly. Aquatic plants live under peculiar conditions. 

 Being wholly or in great part immersed in water, instead 

 of air, the supply of water and dissolved salts is available 

 to any part of the plant capable of absorbing them. 

 Roots then become useful chiefly as anchors. In the 

 quiet watei-s of ponds and ditches, even anchorage can 

 be dispensed with, and we may find, as in the pretty 

 Bladderwoi-ts, or the Ivy-leaved Duckweed, that roots 

 have been altogether dispensed with ; the plant fomis a 

 tangled mass of delicate stems among the other aquatic 

 lierbage, or drifts freely about on the surface ujider 

 the influence of the wind. In certain exceptional cases, 

 roots are used for purposes quite foreign to tlieir usual 

 functions. The Ivy, for instance, so long as it has a 

 wa.ll or tree-trank to cling to, sends out fi-om that side 

 of its stem which is in contact with its support innumer- 

 able short roots, which fasten themselves so closely to 

 the substratum that they will often drag with them scales 

 of bark or pieces of plaster rather thaai relax their hold. 

 These roots ai'e generally used solely for the purpose 

 of clinging, and the plant has in addition a well de- 

 veloped root-system in the soil below for the absorption 

 of water; but they are true roots, and if on a damp wall 

 we sever the stem, the upper part will continue to grow, 

 fed by the clinging roots, which now take up the 

 additional duty of supplying water. 



Roots may even take on themselves the usual woi-k 

 of leaves, and develop chlorophyll, for the production, 

 from the raw materials, of plant-food, in the presence 

 of sunlight. The common Lesser Duckweed sends from 

 the under surface of its floating fronds a bundle of 

 little spirally twisted i-oots which hang down into the 

 water, and help in no small measure to anchor the 

 plant in its unstable substratum. These roots contain 

 chlorophyll, and, being continually exposed to daylight, 

 assist the green parts of the plant in canying on that 



portion of the work which is in most cases performed 

 by the leaves. But most roots, being buried in the 

 ground, could make no use of chlorophyll, which 

 can only fulfil its function in the presence of sunlight, 

 and hence roots are usually not green. 



In the case of the roots of most of our common 

 leguminous plants we have, as Mr. Peai'son has explained 

 in Knowledge for October, 1900, a true symbiosis, or 

 association of two separate organisms for their mutual 

 benefit. 'In many other cases we find parasitism pure 

 and simple, the advantages being entirely coirfined to 

 one side. The curious Toothwort, for instance, has roots 

 which, instead of absorbing nutriment from the soil, 

 fasten themselves to the roots of other plants, penetrate 

 their tissues, and draw therefrom a supply of plant-food 

 ready made. In other cases the parasitism is only 

 pai-tial, as in the Yellow-rattle, Eartsia, and Eyebright 

 of our pastru'es, which, in the keen straggle for mastery 

 which goes on in these densely populated ai-eas, take 

 a somewhat unfair advantage by augmenting the food- 

 supply which they produce by tlieir own exertions by 

 stealing from their neighbours by means of haustoria 

 or suckers developed on their roots, which fasten them- 

 selves to the roots of adjoining plants. To this subject 

 wo shall have occasion to return when we speak of leaves 

 and the part they pla.y in the economy of the plant. 



To tui-n now to stems. The stems of plants have two 

 principal functions. They are the fraanework on which 

 the leaves and flowers are spread out to catch the light 

 and air, and they are the conduits through which the 

 raw food materials are conveyed from the root to the 

 leaves, and the manufactiu^ed plant-food disti'ibuted 

 from the leaves to all parts of the organism. It is 

 with the former use that we shall chiefly concern our- 

 selves. The stems with which we are most familial' are 

 those which rise into the air, generally branching as 

 they go, and thus spreading the leaves and flowers over 

 a considerable space, to allow all to receive theii- due 

 amount of light and air; the form and structure of the 

 stem-system depends directly on the size, nmnber and 

 shape of the leaves and flowers which it is designed to 

 support. But first of all the duration of the stem must 

 be considered. To take a few instances. In forest trees, 

 such as the Oak or Beech, the stem lengthens year by 

 year for a long period of time, branching at frequent 

 intervals. To support such a huge and complicated 

 structure under all circumstances, when loaded with 

 leaves and fruit, or in the stress of winter gales, a stem 

 of great strength and thickness is required. In such 

 plants, as the stem lengthens and branches, it at tho 

 same time increases in girth yea.r by year by means of 

 fresh layers of woody tissue deposited imderneath the 

 constantly enlarging layer of bark ; its sti'ength to resist 

 both compression and bending maintaining a due pixj- 

 portion to the weight and resistance to wind offered 

 iDy the leafy boughs overhead. A great tree-trunk is the 

 result. The trunk has many years in which to perfect 

 its strength, and it becomes by degrees a solid column 

 many feet in circumference of close-grained wood. The 

 conditions are quite different in the case of an annual 

 stem. Here the period of vegetative energy is strictly 

 limited. The stem cannot be begun till winter is past, 

 and must be finished in order that expanded flowers 

 may give place to ripe fruit before winter sets in again. 

 Hence rapidity of consti'uction and economy of material 

 are all-important factors in the plant's building opera- 

 tions. Look at the stem, often four to six feet in height, 

 of a Wild Angelica or Cow-Parsnip, and note how beau- 

 tifullv it fulfils the requisite conditions. Each stem 



