July 1, 1898.] 



KNOWLEDGE 



161 



exposed to the air, the thin cuticle affording little pro- 

 tection against loss by evaporation. 



At a certain depth in most soils underground water 

 exists ; in some cases this approaches the surface, but land 

 plants, as a rule, do not thrive where the soil is in this 

 over-saturated condition : they depend mainly on water 

 precipitated from the atmosphere percolating through the 

 upper porous layers of earth in which their roots are 

 situated. 



From what has now been stated it will readily be under- 

 stood that to plants growing in a region subject to occa- 

 sional drought, any arrangement by which rain falling on 

 the leaves can be quickly transferred to the neighbourhood 

 of their roots must necessarily be highly advantageous. 

 Many provisions of this kind exist. The rain caught by 

 the leaves is not generally allowed to disperse at random, 

 but is led along specially prepared courses. Were the 

 water to lie on the leaves till it evaporated, or were it spilt 

 irregularly, loss and miscarriage would result ; but so accu- 

 rate is the adjustment of plants to their environment that 

 even this apparently insignificant loss is safeguarded by a 

 variety of contrivances. 



The first of these we shall mention is the general inclina- 

 tion of foliage. Many of our readers must have noticed 

 how on a wet day in winter the rain runs down the leafless 

 branches and descends the trunks of trees ; the bark, in 

 consequence of being kept constantly moist, assumes a 

 green tint from the development of mosses and lichens. 

 During summer, on the contrary, the surface of the trunks 

 and branches of most of our trees is quite free from 

 moisture, and the groimd underneath the branches is dry. 

 The foliage, in fact, forms a kind of thatch : the leaves 

 slope outwards and overlap like tiles on a roof, so that the 

 water drips from the outer extremities of the branches, 

 producing a moist zone on the ground around the tree. 

 Now, as the roots commonly extend in a horizontal direction 

 as far as the branches, it is quite obvious they must benefit 

 by this arrangement ; and on extending our observations 

 we find this principle to be of very general application. 



The relative positions of the foliage leaves and absorbent 

 roots are in most cases such as to secure to a greater or 

 less degree the benefits of self-irrigation. The drainage 

 of the foliage may be either centripetal or centrifugal. 

 As examples of the latter, where the slope of the foliage 



most plants with tufted foliage, have the drainage cen- 

 tripetal. The radical leaves of the rhubarb, plantain, 

 dandelion, thistle, and many others, are arranged like a 

 rosette, and the rain llows down towards the central root- 

 stock. In these cases the stem is either prolonged down- 

 wards into a tap-root, or the roots form a central mass of 

 no great horizontal extent. 



With centripetal drainage several secondary adaptations 

 usually appear. Where the inclination of the foliage is 

 outwards, as a rule the leaf-stalk is not channelled ; in 

 plants, on the other hand, which have central roots, there is 

 usually a very conspicuous groove on the upper surface of the 

 petiole. The common cow-parsnip is an exceedingly good 

 example of this ; its long petiole has a deep groove above. 

 The raised edges of the groove almost close over it, and 

 practically convert the leaf-stalk of Heracleum into a 



Fig. 1. — Centrifugal and Ceutripetal Irrigatiou 



is outwards, may be mentioned the lime, birch, apple, pear, 

 plane, maple, ash, horse-chestnut, poplar, and alder. The 

 needle-like leaves of the pine and larch are also so arranged 

 that nearly all the rain is conducted outwards, the ground 

 underneath the branches remaining in consequence remark- 

 ably dry. The drooping or weeping habit of many trees 

 admits of a similar explanation. 



Palms, bananas, tree-ferns ; bulbous monocotyledons 

 like the tulip and hyacinth ; the turnip and beetroot, and 



Fig. 2. — Noddiug aud Auriculate Leaves. 



pipe down which the rain tlows towards the stem. An 

 instructive contrast is seen on comparing an aroid, such 

 as Callocasia or Calladium, which has widely spreading 

 roots and leaves sloping outwards, with the rhubarb, which 

 has a central root-stock, grooved petioles, and inward- 

 sloping leaves. 



The lamina or blade of the leaf in numerous instances 

 exhibits a further pecviliarity. The surface of the veins 

 and midrib may be depressed, as in the primrose, below 

 the general level of the leaf, giving rise to a series of 

 shallow converging channels, which conduct the rain into 

 the grooved petiole. As the raised portions of the leaf 

 have a thin coating of wax, which renders them practically 

 waterproof, the rain falling on these parts assumes the 

 form of globules, and readily rolls off into the courses 

 prepared for it. The wax-coated parts repel water ; 

 but the channels, being entirely free from wax, are easily 

 wetted, so that rain soaks into them and speedily finds 

 its way. Wax or bloom is seen on the leaves of the 

 pea, woodbine, poppy, fumitory, pink, and is present 

 more or less on all leaves. From the leaves of the cabbage 

 water rolls off" as from a duck's back, without wetting the 

 surface. Roots are devoid of wax. On herbaceous plants 

 with an erect stem the leaves diminish in size from below 

 upwards ; the upper ones are either sessile or but slightly 

 stalked ; they may be expanded at the base into auricles 

 or decurrent with a fringe running down each side of the 

 stem. The course followed by the water in its descent 

 depends to some extent on the phyllotaxis. With the 

 opposite arrangement the rain collected by each pair of 

 leaves escapes between the bases of the petioles, and is led 

 down by a groove on each aide of the stem into the axils 



