RELATIONS OF FOLIAGE-LEAVES TO ABSORBENT ROOTS. 95 



it goes, to the earth in the neighbourhood of the absorptive roots, which proceed 

 from the short root-stock. When the leaves of plants furnished with tap-roots 

 are arranged in whorls, and are without internodes, and the rosette rests upon 

 the ground, as is the case in the Mandrake, the Dandelion, and several species of 

 Plantain {Mandragora officinalis, Taraxacum officinale, Plantago media), there 

 are always one or more main grooves on the upper surfaces of the leaves, and 

 the leaves have always such form and position as compel the rain which falls 

 upon them to flow centripetally, i.e. towards the tap-root growing vertically 

 beneath the centre. Plants with petiolate leaves, which conduct rain centri- 

 petally, always have on the upper side of each leaf-stalk an obvious groove, the 

 depth of which is frequently increased by the development of green or (in many 

 cases) membranous ridges on the two lateral edges. Grooves of this kind are 

 to be seen particularly well on the petioles of the radical leaves of the Rhubarb 

 (see fig. 13 2 ), Beet-root, Funkias, and most Violets. 



Far more complicated in structure than the radical leaves just described, are 

 cauline leaves. Leaves proceeding from the stem high above the ground, and 

 forming receptacles for rain-water, like those of the Rhubarb, are best fitted to 

 preserve their proper direction when they have no stalks and the base fits directly 

 on to the stem or passes into it. Cup-shaped laminae, if borne on long erect 

 petioles, necessitate a great expenditure on supporting-cells, and they are, there- 

 fore, on the whole, rare. Of the plants we know, only certain Stork 's-bills, 

 Pelargonium zonale, P. heterogamum, &c, afford examples of cup-shaped, cauline 

 leaves of the kind, borne on long, rigid petioles. In most cases, therefore, cauline 

 leaves which conduct water centripetally are either sessile or very shortly petiolate, 

 have their bases close to the stem, and even extend their edges down it more or 

 less in the form of wings and ridges, or surround it in the form of collars, lobes, 

 and auricles, as in the case of so-called amplexicaul leaves. 



When the leaves are in pairs opposite one another and the alternate pairs at 

 right angles, an arrangement known as decussate, the surplus water is usually 

 conveyed through two grooves, which run down the intervening piece of stem 

 from one pair of leaves to the next. Each of these grooves begins in an indenta- 

 tion between the margins of the bases of a pair of leaves, and terminates above the 

 midrib of one of the leaves belonging to the next pair. Now, water trickling 

 down such a groove falls precisely on that part of a lower leaf where the rain 

 retained by the surface of that leaf is collected; and so the stream of water 

 becomes more and more copious as it approaches the ground. These grooves 

 may be seen in many species of ringent Labiatm, Scrophulariaceat, Primulaceai, 

 Gentianaceos, Rubiacece, and Willow-herbs; the best-marked instances are found 

 in the Knotty Fig-wort {Scropladaria nodosa), the Yellow-rattle (Rhinanthus), 

 the meadow-gentians (Gentiana germanica, Rhcetica, &c), and the Centaury 

 (Enjthraia). The grooves always possess the property of being wetted by water, 

 whereas the ungrooved parts of the same stem are not wetted. Sometimes the 

 grooves are fringed with hairs which absorb the water like the threads of a 



