Nature 43(1105): 207 (Jan 1, 1891)

WHEN in Colorado, I made notes on the plants of the Rocky Mountains, and collected some memoranda from various sources bearing upon alpine floras. Recently, when going over my records to ascertain as far as possible the definite results of environment on high alpine vegetation, I arrived at some conclusions which seemed to throw light on what had puzzled me before. These are at present more or less hypothetical, but I hope to work out the subject in more detail later on, so as to ascertain more clearly whether they will satisfy all the necessities of the case.

High alpine plants differ from those of the lowlands in one or more ways. They may be dwarfed, the leaves more divided or more thickly clothed with hairs, and the flowers are more often blue or pink. For the present, the dwarfing and the colour of the flowers need only be considered. Dwarfing may be—doubtless often is—the direct result of environment, as lack of nourishment. The dwarfed trees grown by the Japanese artificially in small pots are well known.

If this were the only cause of dwarfing, the alpine flora would present clear evidence for the transmission of acquired characters, as the character has undoubtedly become a specific one in several mountain plants. Thus, oaks are normally trees: the Rocky Mountain oak, Quercus undulata, is usually a shrub. The genus Phlox contains fine herbaceous plants, but P. caespitosa, which I have gathered at about 12,000 feet on the Sangre de Cristo Range in Colorado, is so dwarfed as to be called (with other dwarfs growing beside it) "flowering moss." It appears, however, that natural selection may come into play, the low stature of these plants benefiting them in three ways:—

  1. They may escape the violence of the high winds which prevail at those altitudes; taller plants being broken off before the seed matures.
  2. They may obtain some additional warmth from their close proximity to the ground and partial shelter.
  3. The short summer of the mountain-tops necessitates very rapid development, and requires every energy to be thrown into the essential function of producing flowers and seed, leaving nothing to spare for the production of branched stems and diffuse foliage. In short, the plants are obliged to develop as katabolically as possible, and those which fail in the race are ruthlessly cut off by the autumn storms.

The evidence at hand tends to show the reality of this rapid development and the necessity for it. Every alpine traveller knows how rapid is the bursting into bloom of the plants but lately covered by snow and ice. Their impatience, if one may so call it, is astonishing, some plants will begin to develop under the snow. Then the flowers that are produced are very brilliant, lasting but for a short season (like some alpine Lepidoptera), and especially is it to be noted that they are nearly always large in proportion to the size of the plant. That is, the plants have dwarfed, but the flowers have not become dwarfed at the same time. This were strange, if bad nourishment were the only factor: in the light of the facts considered above it is exactly what one might expect.

The intense blue of some high alpine flowers is most noticeable, and the large number of blue flowers is equally remarkable. I never saw anything equalling in blueness the Omphalodes nana var. aretioides of the Colorado mountains above timber-line. The brilliancy of these flowers is almost dazzling. The large and beautiful blue flowers of species of Gentiana, Polemonium, &c., are conspicuous on high mountains. Next to the blues come the pinks. I have shown in the Bulletin of the Torrey Botanical Club how species of Castilleia, which are scarlet or yellow at lower altitudes, become crimson-bracted as they ascend. Many other examples could be given.

I have found no one to dispute the reality of this change of colour, at least in many groups of plants. The cause of it seemed much more obscure. It is generally agreed that, developmentally, reds come after yellows, crimson follows red, and blue crimson. Further, it is supposed that this series of colours is the result of different degrees of pigmental complexity, perhaps of the nature of, or similar to, various degrees of oxidation. Katabolism, strong metabolism, produces the higher colours—the crimsons, the blues. Moisture, slow development, great growth, with an expansion of the parts—these are favourable to the yellows, and, in a less degree, to the reds; of course the green, the primitive chlorophyll, especially.

May not all this, therefore, be correlated with the dwarfing? Strong metabolism is necessary; every energy must be thrown into the inflorescence; and, as a side-result, we get the crimsons and blues of mountain flowers.

It may be, even, that all, or nearly all, blue flowers were produced in this way. Very few of those groups of plants which are never alpine have blue flowers. There will be some exceptions, of course—for instance, I do not know how to account for the blue water-lily—and there are some groups in which a blue flower, however alpine the species, seems to be unproduceable. But even some of the very refractory genera, as Erysimum and Troxinton, do go so far as to produce an occasional purple flower at high altitudes.

If I am right in my suggestions, the need for the selection by bees, made so much of by various authors, no longer exists. My own observations make me very doubtful whether bees do really prefer blue flowers at all, as a general thing—and I can hardly bring myself to believe, though the present hypothesis fall to the ground, that blue in flowers is a result of insect-selection.