116 THE VICTORIAN NATURALIST. 



conceivable that the useful substances existing in the green leaves 

 of the Euphrasia may be transferred to the host plant, and 

 deposited at a convenient time in the permanent part of the root 

 as reserve material, and that in this way the host plant ultimately 

 derives benefit from the so-called parasite." Now this is a 

 supposed case of symbiosis, but on looking further in this direc- 

 tion we can get undoubted cases of " associated existence for 

 the purposes of nutrition," which is Prof. Kerner's definition for 

 symbiosis. 



This association is found most strongly developed amongst the 

 lichens. Take an instance : on the surface of stagnant water 

 may be found small lumps of mucilage, and within these may be 

 observed moniliform rows of round cells, coiled up in the jelly 

 like miniature snakes. This jelly mass is a colony of unicellular 

 algse, termed nostocs. Suppose, now, that the stagnant water be 

 dried up and the jelly mass be broken up into tiny dust-atoms, 

 so small as to be blown away by the wind, and that some of this 

 dust falls upon stones, rocks, or tree trunks. It has been ascer- 

 tained by experiment that the atmosphere always contains an 

 immense number of the spores of fungi, therefore, when rain 

 comes on the spores, the nostoc atoms may lay side by side on 

 the same stone, the moisture acts on both, the atoms swell up into 

 beautiful green cells, and the spores sprout and lengthen into long 

 hair-like filaments, termed hyphae, which wind themselves beween 

 and around the nostoc cells. By repeated division of the cells 

 and by the growth of the hyphse a complete mass of plant sub- 

 stance is formed — this substance is a distinct thallus plant, and 

 is called a lichen. In such a case we find that two totally 

 opposite plants combine to help one another to live, for the thin- 

 walled hyphee are well adapted to absorb moisture and the small 

 quantity of mineral substances required to make cell-sap, and the 

 green nostoc cells are enabled by their chlorophyll, under the 

 influence of sunlight, to obtain carbon from the air in order to 

 turn the inorganic cell sap thus obtained into proper plant food. 

 By this means the algge can multiply the number of its cells, and 

 so increase in volume, whilst the hyphse is enabled to increase as 

 rapidly as the substance of the lichen requires. That this is not 

 mere theory is proved by the following experiment, which was 

 made in a mountain valley of the Tyrol. "A plane surface 

 covered with white filter paper, which was kept moist, was ex- 

 posed to a south wind. In the course of a few hours numerous 

 particles, like dust, adhered to the paper, and amongst them 

 were discovered cell-groups of nostocs, in addition to organic 

 fragments of the most various kinds, such as spores of fungi, &c. 

 These bodies were deposited in the little depressions on the 

 sheet of paper." The above is related in Kerner and Oliver's 

 " Natural History of Plants." Here we see an exact counterpart 

 of what happens in the origin of lichens. It is further stated 



