173 



accumulating at the bottom of the sea ; and as the shallowness of the 

 Bosphorus prevents the general circulation of water from touching the 

 deep layers of the Black Sea cavity, the sulphuretted hydrogen exhaled 

 by the bacteria accumulates and poisons the deeper layers. Quite a new 

 page in the geology of the great interior basins of Eurasia is thus 

 opened. At the same time, a series of new bacteria some of which aid 

 in the production of ammonia in the soil, while others destroy the work 

 of the nitro-monade, are now discovered, and new discoveries are fore- 

 shadowed. 



On the other hand, the benefit derived by higher plants from the 

 lower plants is not limited to the above association with bacteria in the 

 nodules of the leguminosae. Eecent experiments by Th. Schlosing, 

 junior, and Em. Laurent, have proved that various mosses, and espe- 

 cially minute algae (Conferva, Oscillarice, Nitzschea) which usually de- 

 velop on the surface of the soil, also absorb nitrogen from the air. In 

 experiments made in pots, it was sufficient to cover the surface of the 

 mould with a layer of calcined sand to prevent their growth, and at the 

 same time to stop the absorption of nitrogen from the air ; but where no 

 such precaution was taken, nitrogen was absorbed by the algae and 

 the mosses, and after having been assimilated to the soil it went to 

 higher plants. It is also very probable that the leguminosae are not 

 the only plants which can utilise free oxygen from the air with the 

 aid of certain bacteria. Thus, Nobbe and his pupils have lately 

 proved that a shrub of our garden, from quite a different family, nearly 

 akin to the laurel family, namely the Elceagnus angustifolia, also has the 

 same nodules as the leguminosae, which give shelter to bacteria absorb- 

 ing nitrogen from the air of the soil. The little micro-organism is, how- 

 ever, different from the Bacteria radicicola. 



Nitrogen is a* necessary a food for plants as it is for animals. An 

 animal is starved if it does not receive a sufficient supply of nitrogen and 

 its vitality is lowered if it does not have it in an easily assimilable form. 

 The same is true of plants. Insectivorous plants are known to decay when 

 they cannot catch insects ; and the tendency of the day is to recognise that 

 most plants require the aid of some lower organisms for assimilating 

 nitrogen. Thus, B. Frank, who has been working for years in that 

 direction, has proved that the beech can thrive only when a mantle of 

 Mycorhiza-fungi develops over its roots, and that these fungi are not 

 parasites living upon the substance of the roots but real feeders of the 

 beech. They obtain their food from the soil, and while so doing 

 they yield a part of it to the roots of the tree. Further experiments of 

 the same botanist have now shown that the same is true for the pine, 

 which can only thrive in a soil already containing germs of the little 

 fungi, and when its roots become covered with the mantle of fungi, 

 while it leads but a precarious existence in the opposite case. 



All these are evidently but separate instances of a much more gene- 

 ral fact, which only recently became known under the general name of 

 ' symbiosis' and appears to have an immense signification in nature. 

 Higher plants depend upon lower fungi and bacteria for the supply of 

 that important part of their tissues, nitrogen. Lower fungi associate 

 with unicellular algae to form that great division of the vegetable world 

 the lichens. More than a hundred different species of algae are already 



