56 



ECOLOGY 



type, comprises plants whose whip-like 

 stems are almost leafless, and in colour 

 are sometimes pink, sometimes very 

 pale green or white. Since chlorophyll 

 is almost absent, and there are no foli- 

 age Icayes but only a few minute scales, 

 these plants cannot nourish themselyes; 

 morcoyer, the adult plants are not 

 rooted in the soil. Nevertheless they 

 flower freely in dense clusters, and pro- 

 duce large well-stored seeds. Examina- 

 tion of the surface of contact between 

 this root-less climber and the support 

 that it clasps, will show that numerous 

 suckers from the former haye pene- 

 trated the tissue of the host, so as to 

 establish a series of physiological 

 bridges between the two. It is belieyed 

 that these suckers represent highly 

 specialized roots. Their position ap- 

 pears to be determined by contact. The 

 method of attack in Cuscuta europaea 

 is that first a flat adhesive disc projects 

 from the stem of the parasite, attach- 

 ing itself by broadly spreading hairs to 

 the surface of the host. An active 

 growth from within then bursts 

 through the superficial tissue, just as a 

 young root does, and, like a borer, it 

 penetrates the tissue of the host, sink- 

 ing into its tissue softened by a process 

 of digestion. Long rhizoid-like tubes 

 then spread radially out from it, apply- 

 ing themselves especially to the wood 

 and to the pith. Both the conducting 

 tracts and the storage region are thus 

 tapped, and the parasite is put into inti- 

 mate relation with the supplies of its 

 victim. No wonder then that it has 

 need neither of foliage leaves nor of 

 normal roots. In accordance with the 

 general principle of economy these 

 othenvise essential parts are abortive. 

 It has been seen that the dodder 

 seeds profusely; but how does the seed- 

 ling of the parasite initiate the attack? 

 The well-stored seed is able on germina- 

 tion in the soil to start the seedling off 

 in a form suited for success, for its 

 seed-leaves are rudimentary and it pro- 



duces only a simple taproot. Almost all 

 the reserve food is used in forming a 

 long whip-like stem, that moves in the 

 air in wide circles, so as to bring about 

 contact with any living shoot within 

 reach. As soon as this is achieved it 

 promptly becomes a parasitic twiner 

 like the parent. If the food supply runs 

 out before the contact is made, it dies; 

 but if contact be made the root dries 

 oflf, and the plant continues its life in 

 the degraded form shown by the adult. 

 But the degradation is only vegetative; 

 the flowers are normal, though small. 

 The effect of this parasitism on the 

 host is just what one might expect. It 

 may be seen in any clover field in- 

 fected by dodder. The clover is 

 stunted in growth, but it is not other- 

 wise altered unless the parasite actually 

 smothers it. 



Though the adult dodder is thus 

 isolated from the soil, it starts its germi- 

 nation in the ordinary way. But the 

 evergreen mistletoe is an epiphyte 

 from the first, and germinates where it 

 grows, attached to the branches of 

 trees. The viscid white berries, which 

 are one of the sources of bird-lime, are 

 greedily eaten by birds, which wipe 

 their bills on the branches on which 

 they perch to rid them of the sticky 

 seeds. The seeds stick to the twigs, and 

 are thus widely sown. The green em- 

 br}-o, on germinating, protrudes its 

 root, which turning from the light, 

 forms first an attachment disc, from 

 the center of which a root plunges into 

 the tissue of the host. It passes to the 

 level of the cambium, and there estab- 

 lishes with the wood a close relation, 

 which is permanently retained. Other 

 roots arising from it run horizontally 

 through the cortex, and they may give 

 rise to fresh shoots, often at a consider- 

 able distance from the main shoot. 

 This, which springs direct from the 

 plumule of the seedling, develops a 

 forking habit, the pale green stem bear- 

 ing the well-known pairs of yellowish 



