ioo Chapters in Modern Botany CHAP. 



assimilation, but as a secondary function it absorbs mois- 

 ture. Sometimes, as in Tillandsia usneoides, the leaves 

 absorb water through their surface, and then the roots 

 tend to disappear. Another fact of much interest, repre- 

 sented in most collections, but which Grebel describes in 

 detail, is the manner in which some epiphytes, especially 

 ferns, e.g. "the bird's-nest fern" {Aspleniuni nidus-avis) 

 and the "stag's-horn fern" (Platyceriuni), gather nests of 

 humus about their roots, thus literally making soil for them- 

 selves upon the branches. 



Parasitic Plants. The perched plants which grow on 

 the shoulders of their fellows naturally suggest parasites, 

 which to a greater or less extent live at the expense of 

 their hosts. As among animals, there are endoparasites, 

 like some bacteria and fungi, which live within their 

 hosts, and ectoparasites, which, though vitally fixed to 

 their hosts, live outside of them. 



Let us begin with the ectoparasites the outside 

 hangers-on. Some of these, e.g. the mistleto, are only 

 in part dependent on their hosts, the parts which do not 

 pierce the host retaining the ordinary powers of green 

 plants ; others, e.g. the adult dodder, are wholly dependent 

 upon their hosts for the sustenance of their life. 



Mistleto. Of all parasitic plants the mistleto is 

 probably most familiar, as certainly also one of the most 

 remarkable. The species (Viscum album) which grows in 

 Britain and throughout Europe is a parasite on a great 

 variety of trees, such as apple, pear, sycamore, lime, but its 

 favourite host is the black poplar (Populus nigra), and one 

 of its rarest, in Britain at least, the oak. The strange 

 habit of the plant, the beautiful harmony of colour between 

 stem, leaves, and berries, its verdant and fruit-laden con- 

 spicuousness at the turn of the year towards lengthening 

 days and summer, of which the joyous celebration by the 



