904 



THE NATURE BOOK 



these are, of course, produced by the 

 plant, the underlying cause of growth 

 consists in the irritation set up by the 

 minute gall wasps when the insects lay 

 tht'ir eggs in the buds. These particular 

 galls are but types of an immense number 

 of examples exhibiting a wide range of 

 form and size. Moreover, the hfe his- 

 tories of the insects with which they are 

 associated are in many cases imperfectly 

 understood. 



A very singular case of partnership 

 between plants is that which is seen in 

 the case of the Lichens. These used to 

 be regarded as distinct individuals, capable 

 of definite classification on the hnes of 

 other plants. It has been estabhshed 

 that this is entirely a mistaken impression. 

 Researches have shown that the Lichen is 

 not a separate plant at all in the ordinary 

 sense of the word. Rather must it be 

 regarded as a case of that which has been 

 called " symbiosis " — literally, the living 

 together of two very distinct organisms. 

 Every growth of Lichen must be thought 

 of as a colony composed of thousands of 

 individuals, in which, however, there is 

 always one predominant. The chief 

 partner of this strange alliance is a fungus, 

 which encloses in its embrace a countless 

 number of green Algae. 



In a certain sense it may be said that 

 the fungus is parasitic upon these minute 

 organisms, but only to the extent of stimu- 

 lating the Algae to great activity and 

 vigorous increase. Both partners to the 

 alliance appear to receive benefits from 

 thus hving together. Although at first 

 this startling theory aroused an immense 

 amount of opposition, there is no longer 

 any doubt as to its correctness. The 

 most convincing proof was that 

 advanced by Stahl, who after some 

 patient experiments at last succeeded 

 m making Lichens by artificial methods. 

 He took a well-known species of Algae, 

 and on this scattered the spores of a 

 fungus. The result of this unit)n was a 

 Lichen in the form of a well-recognised 

 growth. Again, it is possible to take 

 any of the so-called species of Lichen, 

 and start the constituents of the double 

 organism on a separate existence. It is 

 then seen that the httle cells of the Algae 

 flourish and multiply, whilst the fungoid 

 element of the union as surely starves. 



Now and again it is found that a lichenous 

 growth of one type may embrace several 

 distinct species of Algce — familiar examples 

 which are well known apart from this 

 strange alliance. 



A httle-understood union is that to 

 be seen in the case of Beeches, Birches, 

 and some other trees. Here it is seen 

 that the root tips are infested with a net- 

 work of fungus threads. It has been 

 suggested, although not actually proved, 

 that the threads act as a kind of sponge 

 between the roots and the soil, possibly 

 taking some part in the transmission of 

 moisture. In some instances it is known 

 that the fungus threads really penetrate 

 the substance of the roots, but it does not 

 appear that any harm results. 



In the tropical forests the branches of 

 the trees offer resting-places for a host 

 of plants. These species are entirely 

 isolated from the ground, and were at one 

 time regarded as parasitic in habit, but 

 such is not the case. They lead quite 

 independent existences, carrying out the 

 elaboration of their chlorophyll in the 

 ordinary way. For their moisture and 

 mineral matters these perched plants rely 

 upon rain and the atmospheric dampness 

 which is such a feature of the steamy 

 jungles. It is interesting to note that 

 the roots of most Epiphytes are specially 

 adapted for absorbing moisture from the 

 air, the skin being of a peculiarly spongy 

 nature. There is little doubt that in 

 many cases the debris composed of rot- 

 ting leaves, which collects in the forks 

 of the branches, forms a good rooting 

 medium. A large group of plants in 

 tropical America is formed by the many 

 species of Tillandsia. 



One of the most curious of these, known 

 as "Spaniard's Beard" {T. usneoides), offers 

 an example of a conspicuous and widely 

 distributed Epiphyte. This species has 

 no real roots, but drapes itself round the 

 branches of its host by means of its long 

 thread-like stems of a silver-grey colour. 

 A good proof that the Tillandsias are not 

 in the least dependent upon soil is to be 

 seen in the case of another species of 

 Tillandsia at Kew. This example, which 

 comes from Trinidad, has been suspended 

 from the roof of a greenhouse for a long 

 time. The roots of the specimen are 

 simply in the warm, damp atmosphere of 



