August 1, 1898.] 



KNOWLEDGE 



173 



cavating action ; and at last, in the ordinary course, the 

 sea would have crept up a little at the valley-mouth. But 

 then came the subsidence that has affected our islands 

 so profoundly, accompanied, doubtless, by considerable 

 warping of the old continental floor. Levels were every- 

 where disturbed, and disturbed irregularly ; but the main 

 result on the west coast of Europe, from the " rias " of 

 Spain to the peaked isles of northern Norway, was the 

 admission of the sea into the intricacies of the denuded 

 laud. The lower ten miles of the Erriff valley became 

 converted into Killary Harbour, while the deep clefts in 

 the braes of Bergen admitted the Atlantic for more than a 

 hundred miles. 



It has often, however, been pointed out that, for the 

 production of a true fjord, with its sides free from debris 

 and going down like cliffs into the water, another agent 

 must be introduced. There was a time when fjords were 

 believed to have been excavated, to great depths below 

 sea-level, by the eroding power of glacier-ice. The physical 

 difficulties opposed to this view proved to be considerable ; 

 and dwellers in countries where glaciers are still common 

 have long set their faces against it. But the presence 

 of a glacier in any valley prevents it from becoming choked 

 by detritus from the mountain-walls. The excavating 

 action, which was begun before the ice spread down all 

 the hollows of the country, may still be carried on by the 

 subglaoial streams ; while the ice all the time moulds the 

 valley-walls as it moves forward, and converts each pro- 

 jection in the floor into a characteristic roclw moutonn.'e. 

 Hence, when subsidence occurs, the sea may for some 

 time be banked out of the valley by the presence of the 

 ice. As the glacier shrinks, the sea follows it up the well- 

 preserved groove, in which the only deposits are those of 

 the spreading terminal moraine. For a long period the 

 fjord may thus retain its most typical form ; but at length 

 a delta may spread down from its head, sandbanks may be 

 swept in by the sea, and ordinary taluses may descend 

 upon it and mar the smoothness of its walls. 



Killary Harbour has reached this later stage ; but there 

 is no doubt as to the original prevalence of glacial condi- 

 tions in the district. The whole lower ground of Letter- 

 frack, Tullycross, and Salruck, is ice-worn and mam- 

 millated, and the peat forms as yet only a thin covering 

 across the roches tiioutonnt'cs. The larger of these stand 

 out bare and uncorroded ; and the strise on their surfaces, 

 whether the rock is slate or quartzite or conglomerate, are 

 still marvellously fresh. Probably, as the glaciers with- 

 drew, banks of mud and gravel, washed out from the 

 terminal moraines, covered this lowland in the place of 

 confluent ice ; and the coating that was thus formed helped 

 to preserve the bed-rock from denudation. But here, as 

 elsewhere in our islands, we are led to regard the retreat 

 of the glaciers as a very recent matter. The abundant 

 cirques in the high levels of the moimtains, though not so 

 bare and stern as those of Snowdon, still preserve their 

 outlines, much as when the last ice melted from their floors. 

 One of the latest phases of this old-world highland may 

 have been the most magnificent from a scenic point of 

 view, when the contrasts of crag and snow in Connaught 

 rivalled the glories of Norway or Alaska. 



Even now, are these western mountains of necessity 

 doomed to obliteration: Though the breaches of Mweelrea 

 lie open to the Atlantic storms, and though the grass 

 creeps across the summits, helped by the soft summer rain, 

 may we not read in the long and complex history a tale of 

 regeneration, of vitality rather than decay '? 



* See KsowiEDGB, Vol. XX., p. 210. (September, 1897.) 



SELF-IRRIGATION IN PLANTS.-II. 



By the Rev. Alex. S. Wilsox, m.a., b.si-. 



THE arrangements possessed by plants for collecting 

 and conveying rain to their roots, described in the 

 previous article, derive their value from the cir- 

 cumstance that leaves have but little absorbent 

 power. If greatly parched they will no doubt 

 take up water, but the whole structure of an ordinary leaf 

 is that of an organ highly adapted to the function of 

 eliminating water. Not only are the superficial cells 

 provided with a cuticle through which water can only 

 penetrate very slowly, but moistening causes the stomata 

 to close, cutting off access to the cells in the interior of 

 the leaf. 



Nevertheless, a limited amount of absorption by leaves 

 does occur, and in exceptional cases groups of thin-walled 

 leaf-cells exist which are specialized for this very end. It 

 is principally in species growing under peculiarly adverse 

 conditions, such as shore and desert plants, that marked 

 absorption through leaves occurs. The experiments of 

 Garreau show that the cuticle of many leaves is absolutely 

 impervious to water. This is so especially in old and 

 fully developed leaves. Young leaves, on the other hand, 

 in which cuticularization has not gone far, absorb to a 

 greater or less degree. Washing with soap and water 

 removes wax and increases the absorbent power of leaves. 

 Over the midrib and veins the cuticle is thinner than on 

 other parts of the leaf, and water can penetrate more 

 easily. By far the greater proportion of the absorption 

 takes place, however, at the base of the petiole — in the axil 

 of the leaf, in fact. 



Some of the best established instances of imbibition by 

 leaves occur among plants such as the fuller's teasel, which 

 are provided with leaf-cups. The leaves of the teasel are 

 arranged in pairs ; the broadened base of each leaf unites 

 with that of its opposite neighbour, encircling the stem 

 and forming a receptacle in which a quantity of water 

 collects. That this source supplements the supply furnished 

 by the roots is shown by the fact that cut specimens retain 

 their freshness as long as the leaf-cups are supplied. Leaf- 

 cups of this description are seen in SUphium — one of the 

 gentians — and in a number of other plants. Many epiphytic 

 Bromelias, Tillandsias, and others of the pineapple family 

 retain considerable 

 quantities of water 

 in their expanded 

 leaf-bases, and of 

 this a portion is 

 absorbed by means 

 of certain thin- 

 walled cells. 



These examples 

 have an important 

 bearing on the case 

 of the chickweed, 

 now to be consider- 

 ed. The rapidity 

 with which this 

 plant spreads over 

 garden soil must be 

 attributed in large 

 measure to its very 

 complete system of 

 self- irrigation. 

 The sheathing bases of each pair of leaves on the chick- 

 weed form a kind of leaf-cup where the rain collects. 

 Particles of dust and earth are also washed down into 

 the leaf-axils. Hairs on the margins of the petioles 



Fis. 1. — Leaf-cups of Teasel. 



