under the especial montane conditions. The common upland grass, the sheep’s fescue (Festuca ovina) is thus represented in the montane zone by an allied highland species (F. vivipara) which has the ability to produce young plants in place of the floral structures. This feature is much accelerated by, if not wholly dependent on, the existence of humid conditions, and this is probably the reason why the viviparous form of this plant is found at low altitudes along the seaward margins of Western Britain.
It seems that in order to get some idea of how climatic factors affected upland plants one would have to consider the influence of whole seasons upon the growth of a chosen plant. After making observations upon a number of plants it became clear that there were good practical reasons for using a relatively common plant like the moor-rush (Juncus squarrosus) as material for estimating these effects of altitude. This plant has certain practical advantages for work of this type. It occurs at almost every altitude in Britain and it prefers the wet and base-deficient peaty soils which predominate in the uplands.
The plant consists of a rosette of rather fibrous leaves just above ground level with a long flower-stalk bearing an upper group of brownish flowers or fruits Pl. IX. The latter contain numerous small seeds. At ground level there is a woody stem having numerous roots. The flower-stalks are numerous, they are tough and so can be collected rapidly and transported for subsequent measurement. The fruits, small brown capsules about a sixth of an inch long, are also tough and numerous enough to give suitable numerical measurements. The inflorescence is laid down as part of a bud in summer. It develops the following year, and its length may be taken as a partial expression of the conditions favourable to growth in the preceding summer and
FIG. 15.—Effect of altitude on moor-rush, Juncus squarrosus: L, Length of flower-stalk; N, Number of flowers produced; R, Number of mature capsules.
also in the summer in which it has developed. These conditions affect reproduction in addition by controlling the number of flowers and, later, of fruits and seeds. The only method by which the plant is distributed is by the numerous small seeds.
If one studies the performance of such a common moorland plant at different altitudes, it is apparent that the amount of growth and the production of flowers, or better still, of fruits and seeds, both diminish as the altitude increases (see Fig. 15). But fruit production is affected far more than growth in length, so that a point is reached, generally about an altitude of 2,500 ft. to 2,700 ft., above which fertile fruits are not usually produced, although the plants may form inflorescences of considerable size and in other ways be capable of making satisfactory vegetative growth.
This effect is evidently due mainly to the retardation of the development of the flowers and fruits. Thus in the Lake District in 1942, flowering was completed during June at 700 ft., but it had not begun at the end of July at 2,000 ft., and, at 2,500 ft. to 3,000 ft., it was not complete by the end of August. Thus at these highest levels there was little or no chance of most of the fruits becoming mature and they did not in fact do so. Again, in late September, 1943, only one mature capsule per 20 plants was found on the summit of Ingleborough (2,373 ft.). These and similar facts thus suggest that viable seeds are not usually formed above about 2,500 ft. to 2,700 ft., although large and healthy plants can be found up to at least a thousand feet higher. Until 1947, viable seeds had not been collected from above 2,700 ft., but the exceptionally long and warm summer of that year led to very abundant seed production—so much so that viable seeds were obtained from 3,400 ft., on Ben Wyvis.
In view of the infrequency with which such seeds are formed at high levels, the presence of moor-rush plants at 2,700 ft. and upwards is interesting. They are certainly very long-lived (twenty years or more) and possibly originally due mainly to transported seeds. It is noticeable on some mountains that the plants are not only sporadic but also are often collected in colonies, suggesting a group of individuals centred round a parent plant which has fruited only at rare intervals. The fruits are, perhaps, distributed in the wet wool of sheep, for, as far as is known, no mammals eat the inflorescences although snow-buntings habitually eat the dry fruits in winter and so may help to disperse seeds. The rush is commonest on sheep-infested mountains, and although it occurs to at least 3,700 ft., I have looked for it in vain on the high and grassy Scotch summits where deer habitually graze.
However, it seems certain that the effects of altitude are differential, affecting the seed-production most, flower-production less and vegetative growth least. The analysis of these effects shows that they vary little as between districts receiving great differences in rainfall, and they can thus be attributed mainly to the diminution of mean temperature with increasing altitude. Thus temperature, though it actually operates by controlling the relative rates of development, affects the distribution mechanism.
It is interesting to carry this problem a little further by considering how these things affect a little rush-moth, Coleophora caespititiella (see Pl. 30), that lives in association with the moor-rush and also with the common rush. Its life-history is not very well known, but moths are mature and the eggs are apparently laid in June–July, on or near the flowers of the rush. The larvae then feed on the growing seeds inside the developing fruit. By about the end of August, the infection of a fruit capsule becomes noticeable because of the presence of the larval case, a small cylindrical and white papery object in which the larva may live (see Pl. XI). The larvae, possibly usually with the case, leave the rush-heads in late autumn and hide in the surrounding vegetation until the following summer. With certain obvious precautions, the presence or absence of the white larval cases can be used to study in an approximate way the extent to which the population of heath-rush is infected by the moth. The data also give a picture of the altitudinal distribution of the moth. This is much more restricted than is that of the rush on which it lives. In the central Lake District, in 1942, the frequency of the larvae decreased rapidly from a maximum infection of about 40 per cent of the capsules at 700 ft. and no signs of the moth were seen above 1,800 ft., although in that district the moor-rush goes up to 3,000 ft. Now at first it was thought that the larval cases might become more frequent at a higher level later in the year. In fact larval cases were never seen above this level except in the abnormal summer of 1947, when some were found at 2,000 ft. on the south-facing slopes of Saddleback.
It seemed obvious at first that at higher altitudes the lower temperatures would retard the development both of rush-flowers and of the moth growth-cycle, for both last a year. When no infection was found above 1,800 ft. it was thought that the lower average temperatures might so retard the development of the larvae from the egg to the case stages, that the cases were not produced at higher levels even although there was infection. In this case the larvae might fail to over-winter or the whole growth-cycle might take two seasons. However, no evidence of a later infection at higher levels could be found.
A possible alternative explanation was that, as suggested earlier, the whole growth-cycle of the moth might get “out of step” with that of the rush, so that mature moths and “infectable” rush-flowers (i.e. in the young stage when they are infected) might not coincide in time.
This does, in fact, happen, though not quite in the manner expected. It was found, in samples from the higher levels, that only the early maturing fruits were infected by Coleophora. It followed that there was normally no infection above 1,800 ft. because no rush-flowers were normally open in July above that altitude (1944 and 1945). Even in the abnormal summer of 1947, no sign of infection was seen above 2,000 ft. (and this on a south slope) in the Lake District, and in the Eastern Highlands (Ben Wyvis and Rothiemurchus district) none was noted above 1,400 ft. On the whole, then, it seems as though the main population of mature Coleophora individuals comes out at one time, about June–July. It may then infect any rush-flowers which are then open. This severely limits its altitudinal range, for as we have already seen, the high-level flowers are not mature at these early dates. One difficulty about these findings is that there seems no reason why the cycle of development of the moth should not be retarded somewhat at the higher levels just as that of the rush-flowers is. If this were the case, a small