74 THE BIOLOGY OF FLOWERING PLANTS 



and only slightly hollowed, and are pressed close to the bark 

 of the tree. Adventitious roots grow out under them and 

 are at least kept moist by the sheltering leaves. 



Absorptive Hairs. — In all cases dealt with so far the 

 absorption has been carried out by the roots. We may now 

 turn to plants in which the leaves take up water. The most 

 specialised cases of this are found in the Bromeliaceae, a 

 tropical American family of which the pine-apple is the most 

 familiar example. In Tillandsia regina and Tillandsia unca, 

 which live in the soil, especially in rocky places, and Tillandsia 

 bulbosa which is epiphytic, the sheathing bases of the 

 sword-shaped leaves form a funnel in which a considerable 

 amount of water collects, along with dust, humus, and a whole 

 diversified fauna and flora of algae, frogs and their larvas, 

 spiders, and even httle snakes. " Like watertight tanks," 

 writes Schimper, ** they collect rain-water of which a full 

 litre may descend from one of the larger forms on to a 

 careless collector." This water is used by the leaves, the 

 actual absorption taking place through specialised hairs. 

 These have a stalk of tv/o or three cells sunk in a depression 

 in the epiderm, and an expanded umbrella-like plate or 

 shield of cells for a head. The marginal cells of this head 

 are often expanded so as to form a wing. The outer 

 walls of the shield are frequently much thickened but have 

 no cuticle, or only a very thin one. The shield cells are 

 dead and empty ; those of the stalk living. If not supplied 

 with water the hair collapses and closes the depression in 

 which the stalk arises, the empty cells of the shield preventing 

 excessive evaporation. When water is available it is readily 

 sucked in by the shield cells which expand ; as the stalk 

 cells absorb water they become turgid and swell, so that the 

 shield rises slightly over the depression, and the supply of 

 water to the stalk cells and so to the mesophyll of the leaf 

 is increased. The inner (upper) side of the basal region of 

 the leaves is thickly beset with these absorbing trichomes, 

 and the water requirements of the plant are fully covered. 

 The poorly developed root system serves only for anchoring ; 

 it cannot alone supply the plant's needs. The pine-apple 



