MORPHOLOGY 43 



contact. The climbing roots of many Orchids, Aroids, and Ferns branch 

 and form lodgment places for humus ; and into this the nutritive root 

 branches penetrate as special outgrowths of the climbing roots. Pendent 

 aerial roots generally contain chlorophyll. In the Orchid Angraecum 

 globulosum the task of nourishing the plant is 

 left entirely to the aerial roots, which are 

 then devoid of a velamen, and very much 

 flattened. They are distinctly green-coloured, 

 and supply the place of the leaves which lose 

 their green colour and are reduced to scales. 

 The aerial roots of the epiphytic Bromeli- 

 aceae are developed exclusively as climbing 

 roots, while the leaves function not only as 

 assimilating organs, but also assume the 

 whole task of water -absorption. All the fig. a.—orchu latifoUa. v, The 

 aerial roots of Epiphytes are, so far as their old root-tuber -r the young 



... 1 1 ± ^ . . root-tuber; o, floral snoot; s, 



origin is concerned, adventitious. scale lea( with axmary b ud, 



/.■, from which the new tuber 



The numerous adventitious roots which form has arisen • r > ordinary adven- 

 .... , . , , „ „ titious roots. (A nat. size.) 



a thickly-matted covering on the trunks ot iree- 



ferns become hard after death, and serve as organs of protection. In some 

 Palms (Acanthorrhiza, Iriartea) the adventitious roots on the lower part of the 

 stem become modified into thorns, root-thorns. The roots of certain tropical 

 plants, such as Pandanus and the swamp-inhabiting Mangrove trees, are specially 

 modified. These plants develop on their stems adventitious roots, which 

 grow obliquely downwards into the ground, so that the stems finally appear 

 as if growing on stilts. The Banyan trees of India {Fious Indica) produce wonder- 

 ful root-supports from the under side of their branches, upon which they rest 

 as upon columns. The lateral roots of certain Mangrove trees become modified as 

 peculiar breathing organs, and for this purpose grow upwards into the air out of 

 the swampy soil or water in which the trees grow ; they then become greatly 

 swollen or flattened, and provided with special aerating passages. Such respira- 

 tory or aerating roots surround the Mangrove trees like vigorous Asparagus 

 stalks, and enable the roots growing below in the mud to carry on the necessary 

 exchange of gases with the atmosphere. 



The roots of parasites usually undergo a far-reaching reduction. 

 The roots of the Dodder (Cuscuta) form wart-like excrescences (Fig. 

 185, H) at the point of contact with their nourishing host, which 

 they finally penetrate. They draw nourishment from the host plant, 

 and are consequently termed suction roots or haustoria; such 

 haustoria divide within their host into single threads, and from each 

 thread a new parasitic plant may be formed. The immense flowers of 

 Rafflesia Arnoldi, which spring directly from the roots of Cissus, owe 

 their origin to similar haustoria. The reduction of the roots may 

 extend to such a degree that, in many plants, no roots are formed. 

 It has been already mentioned (p. 23) that in the case of Corallior- 

 rhiza innaia (Fig. 22) the rhizome assumes all the functions of the 



