SECT. I MORPHOLOGY 49 



ground, differ considerably in their structure from underground roots. 

 Tiie aerial roots of the Orchidaceae and of many Aroideae are provided 

 with a spongy sheath, the velamen, by means of which they are 

 enabled to absorb moisture from the atmosphere. Aerial roots, in 

 some cases, grow straight downwards, and upon reaching the ground, 

 branch and function as nutritive roots for the al»sorption of nourish- 

 ment ; in other instances they turn from the light, and, remaining 

 comparatively short and unbranched, fasten themselves as climbing 

 ROOTS to any support with which they come in contact. The climbing 

 roots of many Orchids, Aroids, and Ferns l^ranch and form lodgment 

 places for humus ; and into this absorbent branches of the climbing 

 roots penetrate. Pendent aerial roots generally contain chlorophyll 

 in their cortical tissue. In Orchids belonging to the genera Angraecum 

 and Taeniophijlluiii, 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 flat, dorsiventral, chlorophyll-containing roots of the tropical 

 Podostemaceae C'''), which have Ijeen referred to with regard to the 

 sef{uence of shoots (p. 29), fulfil a similar function. The aerial roots 

 of the epiphytic Bromeliaceae are developed exclu.sively as climbing 

 roots, while the leaves serve not only as assimilating organs, but also 

 assume the whole task of water-absorption. All the aerial roots of 

 epiphytes are, so far as their origin is concerned, adventitious. 



The numerous adventitious roots which form a thickly matted covering on the 

 trunks of Tree-ferns become hard after death, and serve as organs of protection. 

 In some Pahns (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-inhabitating 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 

 standing on stilts. The Banyan tree of India {Ficus inclica) produces peculiar 

 root-supports from the under side of its 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, and are provided with special 

 aerating passages. Such respiratory or aerating roots surround the Mangrove 

 trees looking like vigorous Asparagus stalks, and enable the roots growing below in 

 tlie mud to carry on the necessary exchange of gases with the atmosphere. (Of. 

 Fig. 193;) 



Reduction of Roots. — There is a general relation between the 

 degree of development of the leaf-surface and of the root-system. 

 In saprophytic and parasitic plants, the leaves of which are as a rule 

 more or less comi)letely reduced, a corresponding reduction of the 

 root-.system can be recognised. Special absorbent organs or HAUSTORIA 

 are often developed on the roots of parasites, e.g. on the roots of the 

 green partial parasites Euphrasia, Odontites, Thesium, etc., which become 



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