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, ü.e. towards the tap-root growing vertically 
bereath 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. 137), 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 lamins, 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, &e., 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 torm 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 Labiate, Scrophulariacee, Primulacee, 
Gentianacee, Rubiacee, and Willow-herbs; the best-marked instances are found 
in the Knotty Fig-wort (Scrophularia nodosa), the Yellow-rattle (Rhinanthus), 
the meadow-gentians (Gentiana germanica, Rhetica, &e.), and the Centaury 
(Erythrea). 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 
