6i6 SCIENCE PROGRESS 



thick snow-covering, the plants are protected during the winter 

 from frost, wind, etc. In the summer, on the other hand, the 

 plants are fully exposed to any gales, snowstorms, frost, and 

 other violent atmospheric effects. There are various means 

 of protection against these. One of the commonest, and one 

 which is characteristic of many Spitsbergen plants, is the 

 adoption of the " cushion " habit. The stems branch profusely 

 and keep very short, while the leaves are borne closely together. 

 A compact cushion is thus formed in which the branches 

 mutually protect one another. Debris of various sorts collects 

 in the interstices, adding thereby to the protective effect. The 

 moss campion {Silene acaulis), several of the saxifrages, and 

 the species of whitlow grass {Draba) are excellent examples of 

 this. In some species, e.g. the Arctic willow, a further pro- 

 tection is afforded by the burying of the branches below the 

 surface of the ground or in clumps of moss or lichen. It is 

 noticeable that plants of a given species may adopt a cushion 

 habit in exposed places and a creeping open type of growth in 

 more sheltered spots. A further protection, especially against 

 high winds or sand-blasts and moving substrata generally, is 

 the production of very long, strong tap roots. This develop- 

 ment is also found in situations in temperate countries where 

 the substratum is mobile, e.g. sand dunes, shingle beaches, etc. 

 In Spitsbergen, owing to the open nature of the plant covering, 

 and the extensive denudation, such mobile substrata occur 

 almost everywhere. 



As a result of the strong winds and the continuous daylight, 

 together with the absence of any shelter, transpiration (the 

 passage of water-vapour out of the plant) is very high. Various 

 modifications of the plant structure are developed to cope 

 with this, and to reduce loss of water as much as possible. 

 The " cushion " habit already mentioned is valuable in this 

 connection. There are also rolled leaves, coverings of hairs to 

 the leaves, and succulent leaves. Rolled leaves in which the 

 stomata are enclosed within the roll are not well represented. 

 The cross-leaved andromeda is almost the only representative 

 at all common in Spitsbergen. Hairy leaves in which the 

 transpiring surface is covered by a thick felt of hairs are rather 

 more frequently met with. Species of Draba and chickweeds 

 are good examples of this type of protection. Succulent plants, 

 which, as is well known, grow in places where the transpiration 

 is high, are represented by the rose root (Sedum) in Bear 

 Island and the purple saxifrage. However, the succulence in 

 Arctic plants is never so marked as in desert and steppe species, 

 possibly owing to the freezing effect of the low temperature 

 and the consequent destruction of watery tissue. The modifi- 

 cations mentioned above can all be observed in quite different 



