658 



NA TURE 



[NOVEMBI l: 3, 1923 



The relation between the fungi and orchids varies 

 in different groups of these plants. In some cases 

 symbiosis is intermittent, in others continuous. In 

 Neottia nidus-avis the symbiotic condition is maintained 

 throughout the life-cycle of the orchid, the fungus 

 being found in the roots, rhizome, and even in the 

 flowers and seedsi, and it is transmitted hereditarily. 



The Origin of Tubers in Various Plants. — The occur- 

 rence of endotrophic Mycorhiza in the roots of species 

 of Solanum has been recorded by various observers. 

 Experimenting with the potato, Molliard (1907, 1920) 

 found that tubers were not formed in aseptic cultures. 

 Magrou (192 1) placed potato seeds in a poor soil and 

 close to S. dulca-tnara, which always contains fungi, 

 and found that only when the fungus invaded the 

 potato plant were tubers formed. 



Magrou also investigated tuberisation in Orobus 

 tuber osus (L,eguminosae) and in Mercurialis perennis 

 (Euphorbiaceae), and from his collective studies con- 

 cludes that — 



(i) When the potato plant and Orobus are raised 

 from seed, the establishment of symbiosis leads to 

 tuberisation of the sprouts at the base of the stem ; 

 tubers are not formed in the absence of symbionts. 

 (2) Owing to developmental differences between the 

 two plants, symbiosis in the potato plant is inter- 

 mittent, whilst in Orobus it is continuous. (3) It 

 follows that these plants may develop in two ways : 

 (a) when they harbour symbionts they produce 

 perennial organs ; {b) without symbionts they are 

 dev^oid of perennial organs. (4) It is the rule for wild 

 perennials to harbour symbionts, as Bernard has 

 stated, whilst annuals are devoid of symbionts ; three 

 species of annuals {Solanum nigrum, Orobus coecineus, 

 and Mercurialis annua) may be penetrated by endo- 

 phytes, but they quickly digest the intruders. (5) 

 These observations confirm and supplement the view 

 held by Bernard that tuberisation is due to the 

 association of fungi with plants. 



Mycorhiza in Ericacece, Club -mosses and Ferns. — 

 Rayner (1915-16) finds that Mycorhiza are constantly 

 present in heathers. He isolated Mycorhiza (of the 

 genus Phoma) from Calluna vulgaris, in which the 

 fungus is widely distributed, being found in the roots, 

 branches, and even in the carpels, so that it occurs 

 within the ripe fruit and seed tegument. Calluna 

 seeds, when grown aseptically, give rise to poor little 

 plants devoid of roots, but, under like conditions, in 

 contact with Phoma the plants develop normally and 

 form" many roots. 



In Lycopodiaceae (Club-mosses) and Ophioglossaceae 

 (Ferns), according to Bernard, the perennial prothallus 

 is infested, and the spores whence the plants emanate 

 will not germinate except (as with orchid seeds) with 

 the help of fungi. 



The foregoing emphasises the significance of symbiosis 

 in the vegetable kingdom. I will close by mentioning 

 the theoretical deduction of Bernard that vascular 

 plants owe their origin in the past to the adaptation of 

 certain mosses to symbiotic life with fungi. 



II. Symbiosis in Animals. 

 (i) Algce as Symbionts. 

 Aniinals of widely separated groups characterised 

 by their green colour have long been known. Already 



NO. 2818, VOL. I 12] 



in 1849, von Siet)old attributed tlie colour of Hydra 

 viridis to chlorophyll, which, for a i>criod, was regarded 

 as an animal product. In 1876, G^ Entz concluded 

 that the chlorophyll is contained in vegetable cells 



living as parasites or commensals within the an- '-^^ • 



these cells were aptly named zoochlorella by 

 (1881), whilst cells distinguished by their yellow < oKir 

 were sulxsequently called zooxanthella, the latter 

 having Ix'cn first described by Cicnkovsky (187 1) as 

 present in Kadiolaria. Zoochlorella occur mainly in 

 fresh-water animals; zfx>xanthella mainly in marine 

 animals; the symbionts, measuring 3-10 mirron<; in 

 size, l)eing found in many Protozoa, .Sponges, Cf; 

 ates, ('tenophores, Turl)ellaria, Rotifers, 1'. 

 Annelids, and Molluscs. 



Physiological studies upon the relations Ll.. ... 

 animals and symbiotic algse have yielded interesting: 

 results in Protozoa, Ccelenterates, and Turl)ellaria. 



Symbiotic algae are not usually transmitted heredi- 

 tarily,each host-generation being usually infected afresh 

 by algae. Where Protozoa multiply by division the 

 algae pass directly to succeeding generations. Hereditary 

 transmission occurs in hosts that undergo sexual 

 multiplication (as in Hydra viridis). From the circum- 

 stance that in most cases symbiotic algae are not 

 transmitted hereditarily, we may explain the ocra.sional 

 occurrence of alga-free individuals in a species usually 

 harbouring the symbionts. 



Studies conducted on Turbellaria are of special 

 interest. The best-known example of symbiosis in 

 Turbellaria is found in Convoluta roscoffensis, a species 

 that has been well studied by Keeble and Gamble 

 (1903-7). Its larvae are colourless, and infection 

 occurs after hatching. The cocoon, on the day follow- 

 ing its deposition, is already invaded by algae. 



In Vortex viridis symbiosis is not necessary ; in 

 Convoluta it is necessary' for both partners. Mature 

 Convoluta are never found devoid of algae in Nature, 

 The young larva can only feed itself for a week ; as it 

 grows older it becomes infected progressively with 

 algae. There are four periods in the life of Convoluta, 

 wherein the animal lives at the expense (i) of formed 

 substances, (2) of these and alga-products, (3) of alga- 

 products only, and finally (4) of the algae themselves. 

 This constitutes a true evolution in a species from a 

 free existence, depending only on outside sources of 

 food supply, to a symbiotic mode of life, and lastly one 

 merging into parasitism. 



(2) Symbiosis in Insects. 



iVmong insects we find a whole series of progressive 

 adaptations toward an association with micro-organisms 

 of different categories : 



Group I. — The utilisation by insects of micro- 

 organisms cultivated by them outside their bodies. To 

 quote three examples : (i) The larvae of the beetle 

 Xyloteres lineatus (Bostrichidae) form galleries in the 

 wood of pines in which the fungus Ambrosia is cultivated 

 by the larva for food. The beetle is incapable of 

 digesting cellulose. (2) Termes perrieri of Madagascar 

 builds chambers and galleries. The termites collect 

 dead wood, chew it up finely, swallow it, the wood 

 passing unaffected through their intestine and out 

 in the form of small spherical masses (0-5 mm.) which 

 are cemented together as porous cakes. Fungi which 



