48 



BOTANY 



I 



by their root-caps, by the absence of any indications of leaf develop- 

 ment, and by their internal structure. 



The morphology of the tubers of the Orchidaceae is peculiar. They are, to 

 a great extent, made up of flesh}-, swollen roots, fused together and terminating 

 above in a shoot-bud. At their lower extremity the tubers are either simple or 

 palmately segmented. In the adjoining ligure (Fig. 52) both an old (f) and a 

 young tuber (t") are represented still united together. The older tuber has pro- 

 duced its flowering shoot (b), and has begun to shrivel ; a bud, formed at the base 

 of the shoot, in the axil of a scale leaf (s), has already developed the adventitious 

 roots, which, swollen and fused together, have given rise to the younger tuber. 

 Roots of ordinary form arise from the base of the stem above the tuber. 



Metamorphosis of Roots. The aerial roots of tropical EPI- 

 PHYTES C 27 ) differ considerably in their structure from underground 

 roots. The 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 absorption of nourishment ; in 

 other instances, they turn from the light, 

 and, remaining comparatively short and un- 

 branched, fasten themselves as CLIMIJINC; 

 ROOTS to any support with which they come 

 in contact. The climbing roots of many 

 Orchids, Aroids, and Ferns branch and form 

 Fio.52.-o,rM*taft/w,-,,. r The lodgment places for humus: and into this 



old root-tuber; t", the young , , * , , , '. ,. 



root-tuber; b, floral shoot; s, absorbent branches of the climbing roots 

 penetrate. Pendent aerial roots generally 

 contain chlorophyll in their cortical tissue. 

 InOrchids belonging to the genera, Angraecuin 

 and Taeniophyllum, 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 ( 28 ), which have been 

 referred to with regard to the sequence of shoots (p. 29), fulfil a 

 similar function. The aerial roots of the epiphytic Bromeliaceae 

 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 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. 



scale leaf with axillary bud, 

 A-, from which the new tuber 

 has arisen ; r, ordinary adven- 

 titious roots, (i nat. si/.'.) 



