PARASITISM OF THE MISTLETOE. 25 



and lateral roots arise behind it, which also cling to the support. The rhizophores 

 of some Selaginellas, already mentioned, behave in a manner lundamentall}' quite 

 similar : in them also, the true root-nature only appears when they penetrate into the 

 earth. On the other hand, the aerial roots of many tropical orchids, which live on 

 the branches of high trees, are specially organised for life in the air. The aerial 

 roots are here in the first place clinging organs : stimulated by contact \\itli the 

 cortex of the tree, they wind round and become applied fost to it ; at the same time, 

 however, they possess the function of conveying water to the plant, and, where 

 possible, of absorbing soluble matters. The latter is attained by their putting forth 

 root-hairs where they come into contact with a solid body, to which they cling fast. 

 The absorption of water is favoured in these aerial roots by the so-called velamen 

 arising from the outer layer of tissue behind the growing green end : it is 

 several cell-layers thick, and appears as a white spongy covering, since its cells 

 contain air. The cell-walls are capable of imbibing, and are able to absorb not 

 only rain and dew, but e\en the vapour of the atmosphere. The aerial roots of 

 some tropical Aroids also behave similarly, e.g. species of Aiithuriiim, Philodendron, 

 Rhaphidophora, Monsiera, and others. 



As parasitism acts generally in a degrading manner on organisms, and causes 

 degeneration of organs, so also in roots; the more decidedly parasitism makes 

 itself evident in phanerogamous parasites, the more do their roots lose their ordinary 

 typical structure, and at last only amorphous masses of tissue or even isolated 

 cells remain, which have still in common Mith true roots only the property of 

 penetrating into the substratum, and absorbing food. By l)otanical authors such 

 roots are termed hausioria. A few examples ma}' further illustrate what has been 

 said ^ 



In our INIistletoe {Viscum album), which, as is known, lives on the stems and 

 branches of apple-trees, horse-chestnuts, pines, poplars, and other plants, the roots 

 penetrate, it is true, into the cortex and wood, but chiefly, as one may assume, only 

 to absorb water and minerals dissolved in it, i. e. the so-called crude sap, which 

 is being conveyed in the wood of the tree to the leaves ; whether the roots of the 

 mistletoe, losing themselves in the cortex, possibly extract organic matters also from 

 the tree, is uncertain. So much is at any rate established, that the shoots of the 

 mistletoe, unusually well su[)plietl with chlorophyll, produce organic materials inde- 

 pendently; even the roots flourishing in the wood and cortex are rich in chlorophyll, 

 and green ; the parasitism of the mistletoe, as regards its nutrition, is in any case 

 oiily partial, and, accordingly, also the degrading influence of the parasitism but 

 insignificant. 



If a viscid mistletoe berry sticks to a young branch of a tree, covered with a 

 thin cork layer, the somewhat large embryo, very rich in chloroplull, germinates. 



' The most important treatises concerning phanerogamous parasites are: Franz Unger, äv'- 

 ti-äge zur Komiiiiss der parasitischen Pflanzen ; Eichler, Die BalanopJiorcen, in Flora Brasiliensis, 

 Heft. 47, 1869 ; Graf zu Sohiis-Laubach, Ucbcr Bau und Entiuicklung der Ernährungsorgane 

 parasitischer Phancrogamcn, in Jahrb. f. Wiss. Bot. VI. p. 509 ; the same, Ueber den Thalliis 7>on 

 Pilostyles, Bot. Zeit. 1874, Nos. 4 and 5 ; the same, Die Entwicklung der Blüthc hei Bruginansia, 

 Bot Zeit. 1S76, p. 449; Ludwig Koch, Die A'/re und P/t/chsscid; Heidelberg, 1880; Robert 

 Harlig, L'/icr I'lsr/////. Zcitsclir. f. Korst- und Jagdwesen, Bd. VIII. 



