266 Professor J. Reynolds Green [Feb. 9, 



cells divide and multiply, the hypha? keep pace with them and the 

 lichen is gradually formed. Filamentous algae, such as Scytonema, 

 enter into the composition of lichens in the same way. 



When once formed the lichen can reproduce itself in a way 

 which obviates the necessity for continual reconstruction. In the 

 interior of the old thallus, generally near one surface, particular 

 groups of the two constituents become separated from the rest. 

 Each group consists of a few algal cells wrapped round by hyphal 

 filaments. By the rupture of the thallus these collections, known 

 as soredia, are liberated and grow into fresh lichen plants. 



When we consider the physiological peculiarities of this associa- 

 tion we find there is a very complete and satisfactory division of 

 labour. The fungus is able to condense aqueous vapour, which is 

 very necessary in the dry situations lichens occupy. It tbus provides 

 a solvent for much of the dust and other debris of its resting place, 

 and having effected its solution absorbs it into the hypha and con- 

 ducts it into the interior of the plant. This solution and absorption 

 is facilitated by its power of excreting particular substances, such as 

 certain vegetable acids. It tbus carries raw material to the con- 

 structive algal cells of the interior. The fungus also secures the 

 adhesion of the thallus to the substratum. The alga, on the other 

 hand, by nature of its chlorophyll or green colouring matter, is able 

 to construct food from the raw materials presented to it. It can 

 absorb and decompose the carbon dioxide of the air, and when sup- 

 plied with water and dissolved mineral substances, furnished by the 

 fungus in the way described, can build up carbohydrates such as 

 sugar. Both partners can no doubt take part in tbe processes con- 

 nected with nitrogenous metabolism. 



It is noteworthy that in such an alliance the algal cells grow 

 more vigorously and become larger than similar ones which have 

 no symbiotic partner. 



A case of symbiosis which is more deeply interesting on account 

 of its wide-reaching economical importance, is afforded by various 

 plants belonging to the Leguminosse, the pea, bean and clover family. 

 In order to make clear the special phenomena which these plants 

 present, it is necessary to diverge for a few moments to speak of a 

 feature of vegetable protoplasm. 



The food of plants is derived eventually from extremely simple 

 bodies, which undergo a process of construction into more elaborate 

 ones in tbeir tissues before they can serve actually as food. The 

 simple bodies absorbed are chiefly carbon dioxide from the air, water 

 and various inorganic salts from the soil, the former being taken up 

 by the leaves and other green parts, the latter by the root-hairs. 

 From these simple bodies complex ones, among which especially 

 must be mentioned sugar and proteids, are formed in the cells of 

 the plant, conspicuous among which are those containing chlorophyll 

 bodies or chloroplasts. Among the constituents of proteid matter, 

 which is an absolutely essential part of the food of every living 



