174 



KNOWLEDGE. 



[August 1, 1898. 



Fio. 2— Irrigation of tlie Chick- 

 weed. B, Kootlet. 



help to retain the water, so that the quantity detained 

 at the nodes is greater than one would expect ; indeed, 

 these little reservoirs, relatively to the size of the plant, 

 have, perhaps, quite as large capacity as the leaf-cups 

 of the teasel. The leaf-stalks are channelled, but instead 

 of grooves the chickweed stem has a line of hairs placed 

 on one side, which conducts the overflow from one leaf- 

 cup down to the next, so that after a shower all the 

 leaf-cups are replenished. These hairs on the stem are 

 deflexed, easily wetted, and are evidently arranged to act 

 as rain conductors. 



Each hair consists of several cells which still retain 

 their protoplasm. In a dry 

 condition the walls of the 

 basal cell present a striated 

 appearance, and this has 

 led Kerner to assign to the 

 hairs an absorbent function. 

 But this explanation is un- 

 satisfactory. In the first 

 place, precisely similar 

 striations appear on the 

 leaves and stem if the plant 

 be somewhat dry. Again, 

 the hairs have no direct con- 

 nection with the vascular 

 system, from which they 

 are separated by a thick 

 cellular cortex. Cut ofl' 

 from the internal circulation their power of transmission 

 must be very limited, and whatever water they absorb is 

 quite likely to be lost again on the air becoming dry. The 

 evidence of special adaptation is, at least, not conclusive ; 

 moreover, a circumstance overlooked by Kerner seems to 

 render such special adaptation superfluous. From many 

 of the leaf-axils of chickweed one or two little rootlets, 

 emerge. Even where none are visible a transverse section 

 of the stem reveals their presence beneath the epidermis. 

 It is a very reasonable supposition that the arrangement 

 of conducting hairs and leaf-cups in the chickweed, by 

 which its nodes are kept moist, is designed to promote the 

 formation of these lateral rootlets. Gardeners, at least, 

 when they wish to induce the formation of roots, often 

 adopt a similar expedient. In propagating certain plants 

 recourse is had to iiunrotiiq/e, or the application of moist 

 earth to the base of a branch to stimulate the development 

 of roots. Sir Joseph Hooker, in his llintalaijitn .loumnls, 

 states that the roots which descend from the boughs of the 

 banyan tree are induced to sprout by wet clay and moss 

 tied to the branches, underneath which a little pot of water 

 is hung. So dense is the foliage that the ground beneath 

 the branches gets very dry and hard, the descending roots 

 are unable to penetrate it, and the natives assist matters 

 by conducting the roots through bamboo tubes and by 

 breaking up and moistening the soil at the points where 

 they enter it. At first these roots are very slender, but 

 they soon swell and tighten from the rooting part dragging 

 down the airial. 



The water and particles of earth that accumulate in the 

 leaf-cups of the chickweed not only conduce to the 

 formation of roots — the conducting hairs serve to water 

 them after they are developed. Each rootlet is most con- 

 veniently placed to catch the rain descending from the 

 leaves ; indeed, were it placed under a running tap the 

 position of the rootlet could hardly be more favourable. 

 On the whole, therefore, it seems much more probable 

 that the use of the hairs is to conduct water to the roots, 

 where it is absorbed, than that the hairs themselves are 

 absorbent organs. 



-Transverse Section of Chickweed Stem. 

 H, Hairs. 



If we remove a quantity of chickweed from ground 

 where it has been growing luxuriantly, we are often struck 

 by the remarkable dryness and hardness of the earth. On 

 reaching the soil its rootlets would, therefore, experience 

 difficulty in penetrating were it not that there is an 

 additional point of resemblance to the banyan. The water 

 which drips frequently from the tips of the rootlets keeps 

 the earth soft 

 and moist just 

 at those spots 

 where the 

 rootlets enter 

 it. When they 

 have estab- 

 lished them- 

 selves they 

 drag down the 

 stem ; each 

 internode and 

 each young 

 shoot become Kir. .s. 

 practically in- 

 dependent ; 

 hence the rapidity with which this weed spreads. 



Another circumstance worthy of mention is the obvious 

 relation between the conducting hairs and the axillary 

 buds. Although the hairs are difi'erently placed in suc- 

 cessive internodes, they are always on the same side 

 as the axillary bud below. The latter is consequently 

 drenched from time to time by rain descending along the 

 conducting hairs. As young leaves are to some extent 

 capable of absorbing, the developing shoots must, therefore, 

 participate along with the rootlets in the benefits of this 

 system of automatic irrigation. 



The special necessity for this curious arrangement in 

 the chickweed may possibly arise from the small amount 

 of lignified tissue possessed by the plant. Its rapid growth 

 does not, indeed, admit of much lignification, which is a 

 process requiring time. Not only is the vascular cylinder 

 running up the centre of the stem of small dimensions, 

 but the four lignified strands (dark coloured in the figure), 

 through which the water rises from the roots, are very 

 slender relatively to the thickness of the stem. We 

 might almost compare the chickweed to a house where the 

 pipe from the main is of 

 narrow cahbre, and an 

 additional supply has to 

 be obtained by collecting 

 the rain from the roof into 

 cisterns. As the tendency 

 of cultivation is to render 

 soil dry, this double water- 

 supply is also advan- 

 tageous, no doubt, in 

 relation to the peculiar 

 habitat of the plant. 



Aqueducts consisting of 

 lines of hairs simUar to 

 those of the chickweed 

 occur in a number of 

 plants. The Germander 

 speedwell has a line of hairs on both sides of its stem, 

 and in allied species there may be three or more such 

 lines corresponding in position exactly to the grooves 

 by which rain is led down the stems of so many plants. 

 Those speedwells which occur as garden weeds emit root- 

 lets, and have the same creeping habit as the chickweed. 

 Their delicate transparent rootlets are often exquisitely 

 beautiful, being covered with microscopic fibrils so exceed- 



Fio. 4. — Vertical Section of Chick- 

 weed Stem, with two Rudimentarv 

 Rootlets. The Spiral Vessels sliow 

 the course of the ascending Sap. 



