.216 THE JOURNAL OF BOTANY 



:mate dualities, the whole point is that there is no new factor intro- 

 ,duce(l, either morphological or ph^^siological, beyond a perfect blend 

 >of pre-existing somatic factors, which may separate ont without 

 Ibeing affected in the least in the 'reverted' branches. It should be 

 iinte resting to examine the organization of the consortia, and reduce 

 them to simpler factors. A dual organization, if implying a dual 

 control, is, in fact, a biological futility, as a house divided against 

 itself, amd hence doomed to rapid extinction. Lichens grow and 

 thrive, within their own limitations, and liave done so for indefinite 

 hundreds of millions of years, the time-factor in all such phenomena 

 appearing incredible. The chances are against dual control, (piite 

 apart from the fact that the obvious tendency of all biological evolu- 

 tion, both im zoology and botany^ has been to establish all successful 

 organism AvLth united control, as expressed ultimately in a central 

 nervous system and the life of the individual. On the other hand, 

 mutual advamtage maj^ be more clearly expressed as mutual dependence ; 

 . and in this sense the fungus is interpreted as being dependent on 

 the alga only for diffusible carbohydrate ; while the alga is often 

 : supposed to take salts and water from the substratum via the 

 ihyph?e of the fungus'^ — apparently on the assumption that being 

 ■wholly enclosed within the mycelium it could not get them otber- 

 wise. That the alga acquires a certain amount of shelter from 

 . extreme insolation, or from sudden desiccation, may be admitted ; but 

 ;it is quite unjustifiable to suppose that in the general case it is 

 ^ possible to get anything from a living fungus (!), the primary attack 

 of which is undoubtedl}' quite as much the indication of a search for 

 vw^ater and combined nitrogen, as for the carbohydrate it absorbs 

 parasitically, either by haustorial penetration of the algal cells, ob- 

 served in a few cases 2, or else by stronger osmotic activity 3. Khiz- 

 -oids of benthic algai, or rhizines of lichens, are feeble absorptive and 

 conductive organs at the best for their own thallus. The alga lives, 

 ;as in the free state, entirely on what it can get from atmosi)heric 

 iprecipitations and their swamping effect on the substratum ; but it 

 gains from the disintegation of adjacent dead fungus-hyphse and 

 their residual contents, thus establishing a certain amount of cyclic 

 rotation in the seaaty supply of nitrogen and phosphoros com- 

 pounds. 



It is interesting to compare the accounts of accepted text-books 

 on the subject. An obsession for looking at everything in Botany 

 through Continental spectacles has admittedly characterized the 

 writers of the last few decades in this country, but the same mental 

 attitude may not apjDeal to a younger generation. Thus Sachs 4, from 

 the standpoint of a physiologist, describes the Lichen as * compounded ' 

 of an alga and a fungus ; and yet clearly states that the algae are 

 * imprisoned ' by the latter, heading the page ' commensalism.'' On 

 the other hand, though he sees the problem of the * form ' of the 

 complex — as the symbionts assume forms " otherwise proper only to 



1 Strasburger, op. cH, 



2 Bornet, Ann. Sci. Nat. xvii. 54 (1873). 



^ Paulson in Journ. Linn. Soc. (Bot.) xliv. 503 (1920). 

 •* Sachs, Physiology, Engl. Trans, p. 393 (1887). 



