CONCERNING SYMBIOTIC UNIONS AND INTERACTIONS 177 



plants are capable of symbiotic union, and in each case the causes 

 determining the fusion or non-fusion need to be discovered. The same 

 questions apply to the union between plants of the same kind. Thus 

 the hyphae of certain fungi may unite to form a pseudo-parenchyma, 

 whereas those of other fungi and the filaments of most algae remain free 

 even when brought into close contact. These peculiarities are of as great 

 importance in the maintenance of specific shape as are the differentiation 

 and separation of special cells or organs for particular purposes. 



In the process of grafting, the formation of wound-callus renders 

 possible the fusion of the cell-walls of the symbionts, but this and also the 

 differentiation and fusion of the vascular bundles take place incompletely 

 in the callus of those plants which are incapable of symbiotic fusion l . 



In nature the symbionts are brought together either accidentally, or 

 by chemical and other orienting stimuli. These play an important part, 

 for example, in the penetration of a plant by the hyphae of parasitic 

 fungi, and possibly also in inducing the fusion of fungal hyphae to form 

 pseudo-parenchyma. Chemical stimuli possibly determine the grouping 

 of the zoospores to form the vegetative network of Hydrodictyon utricula- 

 tum 2 , or the regularly shaped coenobium of Pediastrum 3 . Furthermore, 

 chemical stimuli are responsible for the attraction of the pollen-tubes and 

 of the antherozooids of certain plants to the ovum. 



Attraction does not, however, always result in fusion, for the anthero- 

 zooids of all ferns are attracted into any ripe fern archegonium, but only 

 those of the same species fuse with the ovum. Further, the maintenance 

 of symbiosis requires the existence of certain affinities between the two 

 organisms, or at least the fulfilment of certain conditions. It must, however, 

 remain uncertain whether these conditions are already represented by the 

 permanent properties of the fusing cells, or whether they are created by 

 functional interaction or by the action of special stimuli. All these factors 

 are variable in character, and hence it is comprehensible that the copulating 

 tubes of Spirogyra should fuse, but not the actual filaments, and that in the 

 progress of development particular cells may separate from the rest and 

 be thrown off. A permanent union will naturally be impossible when one 

 organism forms a metabolic product which is poisonous or repellent to 

 the other. 



Conjunctive symbiosis may involve either the mere close apposition 

 and growing together of cells, or the penetration of the one symbiont into 



protoplasts of the other, or even an actual sexual or asexual fusion 

 Jtween the protoplasts of the two symbionts. Fusion of the cell-walls 



1 On normal fusions cf. Hofmeister, Allgem. Morph., 1868, p. 548; Goebel, Organography, 

 ), I, p. 51. 



2 Cf. Klebs, Bot. Ztg., 1891, p. 821. 



3 Askenasy, Ber. d. Bot. Ges., 1888, p. 127. 



PFEFFER. II 



