017 



and some of Ihe others, one may presume, that their defeclive fruit- 

 ing is due in some degree to the lack oF the necessary insects. 

 Such an explanation will not suffice, however, for most of the Mo- 

 nocotyledons, bccause they are wind-pollinated (Oichids and Zo- 

 stera excepted), and it seems more probable that the defeclive fruit- 

 formation is due to the low summer temperature, and to the in- 

 jurious effect of the incessant rain on the pollen. 



Anolher effect of these climalic conditions is the tendency of 

 cerlain species to appear as viviparous forms, a faet, already referred 

 to by Rostrup (1870, p. 13). On the Færoes Aira alpiim, Fesinca 

 ouina and Poa alpina occur in viviparous forms only, and with 

 these \ve may put Polygoniim viviparum, Sediim villosiim and Lyco- 

 podiiim selago, which nearly always bear bulbils (compare the bul- 

 biferous species, p. 909). This tendency towards Pseudovi vipary 

 may also be observed in the case of species not generally given to 

 the habit; thus we found viviparous specimens of Aira fle.viiosa 

 particularly in the mountains, and at one place some of Agrostis 

 caniim. 



It is my opinion, that this tendency towards pseudovivipary, 

 together with the lack of fruiting, are due to the action of the in- 

 sular climate in discouraging sexual reproduction and encouraging 

 vegetative propagation. 



6. Distribution in attitude. 



Species in mountainous countries are not equally distributed 

 in the lower regions and up in the mountains. The external con- 

 ditions in the mountains are more rigorous and hinder the growth 

 of many lowland species. On the other hånd certain species find 

 their natural home on the heights, probably because they cannot 

 compete with the lowland species, and are unable to endure the 

 higher summer temperature of the lower zones. It is not, however, 

 the height of any locality above sea-level that determines, which 

 species may thrive there, and which may not. The condition of 

 the soil and the exposure are of great importance. A slope facing 

 south affords, as already stated, conditions (sunshine, etc.) differing 

 from and generally better than a northern exposure or a tlat place, 

 hence piants can thrive at a higher altitude. 



The altitude above sea-level need not be an indication of the 

 limits of any species, and the mere record of how high up any 



59* 



