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PRINCIPLES OP PLANT-TERATOLOGY. 



trophy. When the main root is absent or poorly formed, 

 the lateral roots, by correlative growth, develop in such 

 a way as to supply its place. The fasciated roots 

 represent merely a particularly luxuriant mode of 

 growth, from what cause we do not know. All that 

 will be said in regard to the morphological explana- 

 tion of fasciation in the stem will apply equally to the 

 root ; the phenomenon is the same in both ; hence this 

 explanation will be deferred until the stem is treated of. 



The cases of fusion described are of much value, for 

 they show us clearly the difference between post- and 

 congenital union and fasciation of organs. In the 

 Tecoma- and B/ms-roots the congenital union, Franke 

 points out, is comparable, not to the development of a 

 gamopetalous corolla where the free tips are the first 

 rjarts to be formed, but rather to the leaf-sheath of 

 Equisetum in which the separate teeth are formed 

 subsequently to the common ring- wall. The structure 

 in Tecoma and Rhus is clearly of a compound nature, 

 as the presence of three plerome-cylinclers indicates ; 

 a fasciated root is always a single, uncompounded 

 structure, dividing above, and at the base possessing 

 but a single plerome-cylinder. It is of importance to 

 distinguish between these two phenomena. 



3. CHANGE OF DIRECTION OF GROWTH. 



In the majority of plants the tap-root grows vertically 

 downwards and the lateral roots grow obliquely down- 

 wards or horizontally. There are plenty of plants, 

 however, in which the roots take quite other directions; 

 in this connection one need only mention the apogeo- 

 tropic roots of cycads, of mangroves, and of Taxodium 

 distichum, which grow vertically above the surface of 

 the soil for the purpose of obtaining a better supply 

 of air. Roots can, for the most part, be induced to 

 grow in any direction by artificially changing the 

 nature of the medium in which they grow ; e. g. roots 

 will always grow away from a dry and towards a moist 



