190 



HUTCHINSON'S POPULAR BOTANY 



being due to a papery membrane which envelops the green chlorophyll- 

 containing cells of the true roots. This covering is composed of perforated 

 cells, and acts like a sponge. " "When it comes in contact with water in 

 the liquid state," says Kerner, ' : or more especially when it is moistened 

 by atmospheric deposits, it imbibes instantaneously its fill of water. The 

 deeper-lying living green cells of the root are thus surrounded by a fluid 

 envelope and are able to obtain from it as much water as they require." 



Moreover, this porous tissue possesses 

 the power of condensing aqueous 

 vapours and other gases ; so that a 

 Tree-orchid is absolutely independ- 

 ent of its host for nourishment. 



It will be evident from the above 

 facts that the papery envelope has 

 a twofold use. In the dry season it 

 reinforces the safeguards provided 

 by the root against too profuse 

 transpiration on the part of the 

 living green cells in the interior; 

 " and in the wet season," as Kerner 

 remarks, " it provides for the con- 

 tinuous supply of the requisite 

 quantity of water." The air-roots 

 of many Aroids and Tree-ferns 

 answer much the same purpose; 

 but this is not the case with the 

 peg-like aerial roots of Ivy (Hedera 

 helix), which are simply intended for 

 mechanical support. The nourish- 

 ment required by the Ivy is obtained 

 in an entirely honourable manner 

 by its leaves and underground 

 roots ; and the rather rough treat- 

 ment which the plant has received 

 from some writers on account of its 

 supposed parasitical tendencies is, 

 to say the least, unfortunate. One poet charges it with having " hid the 

 princely trunk, and sucked the verdure out on't " ; but the ''prejudice on 

 which the accusation is based has no foundation in fact. 



The aerial roots of Ivy are, in short, an arrangement by means of which 

 the plant clings and climbs ; and though it is doubtless true that they 

 penetrate into the bark of trees, their object is not plunder, but the obtaining 

 a more secure anchorage. But on the other side, it must be admitted 

 that many a fine tree is killed by the Ivy robbing it of light and air 



FIG. 242. SCKEW-PINE (Pandanus utilis). 



A native of Madagascar, whose aerial roots have cup-shaped 

 extremities. From its saw-edged leaves are made sugar- 

 bags and the familiar " mats " used by fishmongers and 

 poulterers. Height about sixty feet. 



