The milfoil (_Achillea Millefolium_) served as a second example, and the experiments were carried on in the same localities. The long and thick rootstocks of the alpine plant bearing short stems only with a few dense corymbs contrasted markedly with the slender stems, loose foliage and rich groups of flowerheads of the lowland plant. The same differences, in inner and outer structures were observed in numerous instances, showing that the alpine type in these cases is dependent on the climate, and that the capacity for assuming the antagonistic characters is present in every individual of the species. The external conditions decide which of them becomes active and which remains inactive, and the case seems to be exactly parallel to that of the water-persicaria. In the experiments of Bonnier the influence of the soil was, as a rule, excluded by transplanting [442] part of the original earth with the transplanted half of the plant. From this he concluded that the observed changes were due to the inequality of the climate. This involved three main factors, light, moisture and temperature. On the mountains the light is more intense, the air drier and cooler. Control-experiments were made on the mountains, depriving the plants of part of the light. In various ways they were more or less shaded, and as a rule responded to this treatment in the same way as to transplantation to the plain below. Bonnier concluded that, though more than one factor takes part in inciting the morphologic changes, light is to be considered as the chief agency. The response is to be considered as a useful one, as the whole structure of the alpine varieties is fitted to produce a large amount of organic material in a short time, which enables the plants to thrive during the short summers and long winters of their elevated stations. In connection with these studies on the influences of alpine climates, Bonnier has investigated the internal structure of arctic plants, and made a series of experiments on growth in continuous electric light. The arctic climate is cold, but wet, and the structure of the leaves is correspondingly loose, though the plants become [443] as small as on the Alps. Continuous electric light had very curious effects; the plants became etiolated, as if growing in darkness, with the exception that they assumed a deep green tinge. They showed more analogy with the arctic than with the alpine type.