Further knowledge of the social relationships of plant species is worth having when we have got it, may be doubted by some. Perhaps it may be felt that study should be restricted to such plants as we can use directly; or extended at most to those associated with the useful ones. To such objections there are two answers : first, that we never know when any species of living organism is going to have a serious effect on our health, wealth and welfare; second, that it is well to realise that if anything happened to bring to an end the plant-life of this earth, the whole animal kingdom, including our own species, would inevitably and speedily perish. Of that there is not the least doubt. The inhabitant of the great modern city is apt to forget his dependence and it cannot be otherwise than salutary occasionally to remind ourselves of the extent of the interdependence which underlies our own lives and those of every other species of living organism. These considerations should be enough to convince anybody that such studies, the scope of which we have tried to indicate, are more than a pleasant means of passing the time. If more be needed we may consider the words of A. H. Church :

‘So long as the earth goes round the sun, with its axis inclined at a very constant angle with the plane of the ecliptic, seed-time and harvest, cold and heat, summer and winter, day and night, cannot cease in this part of the globe; the main cycle of the annual succession of plant-life being the response to the astronomical organisation of the solar system. Hence the story repeats in successive years with a precision which at once affords the most

mysteriously beautiful feature of the countryside, as it is the most constant factor in determining the main lines of all social organisation. In an age which exploits coal and petrol, it is well to remember that human life is still wholly dependent on living plant-plasma, and likely to remain so indefinitely. The world-supply of coal and petrol is, after all, finite, and the plant remains the simplest and cheapest means of obtaining current supplies of solar energy, as well as being the source of essential food-substances beyond the hope of chemical synthesis.’

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