sect, ii PHYSIOLOGY 213 



while certain Fungi and Bacteria do not require free oxygen for their 

 development (Anaerobionts), others (the so-called aerobiotic forms) are 

 unable to develop or indeed to exist without oxygen. 



While many Fungi inflict far greater injury upon their host-plants 

 by the decomposition they induce, than by the withdrawal of the 

 nutritive substances, others produce a different effect. The Rust-fungi, 

 for instance, do comparatively little injury to their host ; while the 

 relation between host and Fungus in the case of the Lichens has been 

 shown to be absolutely beneficial. The Lichens were formerly considered 

 to be a third group of the lower Cryptogams and of equal value with 

 the Algae and Fungi. It is only in recent years that the astounding 

 discovery was made by De Baey and verified by the investigations 

 particularly of Schwendenee, Beess, and Stahl, that the body of 

 the Lichens is not a single organism, but in reality consists of Algae 

 {e.g. fission - Algae), which also exist in a free state, and of Fungi, 

 which for the most part belong to the Ascomycetes. The Fungus 

 hyphae within the Lichen weave themselves around the Algae ; and 

 while the latter occupy the upper or outer side of the leaf-like or 

 cylindrical thallus as the more favourable position for assimilation, 

 the hyphae come into the closest contact with them and absorb from 

 them part of their assimilated products. The Fungi in return provide 

 the Algae with nutrient water, and enable them to live in situations 

 in which they could not otherwise exist. As a result of this close 

 union with the Fungi, the Algae are in no way exhausted, but become 

 more vigorous than in their free condition, and reproduce themselves 

 by cell division. As both symbionts, the Algae as well as the Fungi, 

 thus derive mutual advantage from their consortism, Lichens form one 

 of the most typical examples of vegetable symbiosis. 



The cause of the regular appearance of the fission-Algae Nostoc 

 and Anabaena in the roots of the Gyaideae and in the leaves of 

 Azolla and other water-plants is much less easy to explain. 



In connection with these cases of symbiosis between plants, mention may here 

 be made of the similar symbiotic relation existing between animals and plants. 

 Like the Lichen-fungi, the lower animals, according to Brandt, profit by an 

 association with unicellular Algae by appropriating their assimilated products with- 

 out at the same time disturbing the performance of their functions. Fresh -water 

 Polyps (Hydra), Sponges (Spongilla), Ciliata (Stentor, Paramecium), also Helio- 

 zoas, Vermes (Planaria), and Amoebae {A. proteus) are often characterised by a deep 

 green colour, due to numerous Algae which they harbour within their bodies, and 

 from the products of whose assimilation they also derive nourishment. In the case 

 of the Radiolarias, the so-called "yellow cells," which have been distinguished as 

 yellow unicellular Seaweeds, function in the same way as the green Algae in the 

 other instances. Another remarkable example of symbiosis in which the relation- 

 ship is not one merely of simple nutrition, has been developed between certain 

 plants and ants. The so-called Axt-plants (Myrmecophytae) offer to certain small 

 extremely warlike ants a duelling in convenient cavities of the stems (Cecropia), 

 in hollow thorns (Acacia spadicigera and sphaerocephala, Fig. 187, N), in swollen 



