1900.] on Symbiosis and Symbiotic Fermentation. 263 



Among the flowering plants a true symbiosis is exhibited by a 

 group, the chief members of which are met with in the natural 

 orders Scrophulariacese and Santalaceas. They are herbaceous plants 

 of small size, which flourish in pastures or other situations where 

 there is a rich development of herbage. Some species are found 

 growing freely in woods. Of these, the little plant known as 

 Thesium humifusum affords a typical example. It grows on rough 

 ground usually among grass, and develops a fairly large root system, 

 which extends to some depth in the soil. When the branches of its 

 roots come into contact with the roots of other plants in the same 

 earth, a little swelling is developed on them at the point of contact, 

 and from this swelling certain outgrowths of the epidermal cells 

 proceed, which penetrate into the tissues of the other root, establish- 

 ing a very intimate relationship between them, so intimate in fact 

 that ultimately the tissues of the two roots become indistinguishable 

 in the zone common to both. By the relationship thus set up, food 

 materials elaborated by one of the symbionts can pass into the other, 

 and thus each can co-operate with the other for the common good. 

 The degree to which each partner benefits is not always the same, 

 but the relationship proves to be mutually advantageous. The asso- 

 ciation of the two always depends upon the root of the Thesium 

 fastening upon that of some other plant. It never receives an 

 attachment in turn. The same thing is true of the members of the 

 Scrophulariaceas which have been alluded to, conspicuous among 

 which are Bhinanthus, Melampyrum, and Euphrasia. On account of 

 this somewhat one-sided way of establishing the symbiosis, these 

 plants are frequently spoken of as root-parasites. It is better 

 perhaps not to employ that term, as the alliance appears to be 

 mutually beneficial, at any rate to a considerable extent. 



An equally interesting instance of a similar relationship is pre- 

 sented by the mistletoe. This plant has been generally somewhat 

 vaguely described as parasitic on the oak, the apple, the poplar, and 

 other trees. The degree of its parasitism has, however, been very much 

 exaggerated, if that term should be applied to the plant at all. The 

 mistletoe always grows from seed which is carried from the parent 

 plant by birds, and deposited upon a branch of one of these trees. 

 The seed germinates where it is dropped upon the branch, and a bulky 

 hemispherical radicle is pressed against the bark. This grows and 

 flattens into a strongly-marked disc, from which a projection is put 

 out which penetrates the soft tissue of the cortex and reaches the 

 wood. Lateral outgrowths from this projection or sinker then grow 

 at right angles and burrow along the cortex of the tree. From these 

 new sinkers again are sent down into the soft tissues. They grow 

 slowly, and as the branch which they penetrate becomes thicker by 

 the activity of the cambium layer, the new wood thus formed sur- 

 rounds the sinkers, and the latter thus become embedded in it. A 

 very complete union of the tissues of the mistletoe and the host is 

 thus established. The mistletoe is a slow-growing plant, and the 



