MORPHOLOGY 49 



In some Palms (Acantkarrhiza, Iriartca) the adventitious roots on the lower part 

 of the stem become modified into thorns, ROOT-THORNS. The roots of certain 

 tropical plants, such as Paiidanus 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 

 standing on stilts. The Banyan trees of India (Ficus indica) produce wonderful 

 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, and are provided with special 

 aerating passages. Such KESPIKATORY or AERATING ROOTS surround the Mangrove 

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

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

 Fig. 213.) 



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 shoots of which are as a rule 

 extremely 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 

 attached to the roots of their host plants by means of disc-shaped 

 or wart-shaped haustoria. In extreme cases no subterranean roots 

 may be developed, and the parasite only bears haustoria which 

 penetrate the host (Cuscuta, Fig. 202 H). The haustoria of the 

 Kafflesiaceae traverse the body of the host -plant as elongated 

 filaments of cells, and are capable of regenerating the parasite. 

 The immense flowers of Rafflesia Arnoldi, which spring directly from 

 the roots of Cissus, originate from similar haustoria. The reduction 

 of the roots may extend to such a degree that, in a number of plants, 

 no roots are formed. It has been already mentioned that in the 

 case of Coralliorrhiza innata (Fig. 24) the rhizome assumes all the 

 functions of the roots, which are entirely absent. Also in many aquatic 

 plants (Scdvinia, Wolffia arrhiza, Utricularia, Ceratophyllum) roots are 

 altogether absent since these plants no longer require them. The 

 beard-like epiphytic Tillandsia usneoides, belonging to the Bromeliaceae, 

 has no roots and obtains the water it requires by means of special 

 scale-like hairs. 



Members of Independent Origin 



The existence of parts of the plant with the characters of 

 distinct primary members, but to which an independent phylognetic 

 origin must be ascribed, has been pointed out by GOEBEL ( 29 ). The 

 tubers which occur among the Dioscoreaceae, an order of twining 

 plants belonging to the Monocotyledons, are of this nature (Fig. 53). 

 The extreme forms of these tubers are flat, cake-shaped bodies, which 

 in development and structure exhibit a mixture of the characteristics 



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