528 MEASUEES FOR PROTECTING GROWING PLANTS FROM LOSS OF HEAT. 



which now no longer spread beyond the narrow circle of their confined habitat, 

 although they ripen seeds capable of germinating, and are met with again in great 

 quantities one or two degrees farther south. We may conclude that these plants were 

 first brought to their isolated habitats within the historical period by wind or other 

 distributive agents, and everything tends to show that they represent the remnant 

 of a vegetation which was distributed very widely over adjacent districts in ages 

 long past, but have withdrawn thence in consequence of the severe climate which 

 has intervened; that is to say, have died and been replaced by other vegetation. That 

 such foundlings on isolated mountain slopes, often only a small steep ravine, or on 

 a single rocky face, could maintain themselves even in the later cold periods, is 

 explained by the fact that conditions of warmth can prevail over very restricted 

 areas on the mountains which difier in toto from those of the environment, and are 

 only found generally prevailing quite a degree further south. The southern slope 

 of the Solstein range, between Hall and Zirl, produces in limited areas Hop-horn- 

 beams and Bladder-senna (Ostrya carpinifolia and Colutea arborescens); from 

 the boulders an umbellifer, the curious Tom.masinia verticillaris, rises to the 

 height of a man; the rock terraces are overgrown with Stipa pennata, Lasiagrostis 

 Galainagrostis, Saponaria ocymoides, Dorycnium decumbens, and here and there 

 one might imagine one's self a degree further south on the other side of the Alps. 

 It is beyond question that the plant forms named on the warmest and most pro- 

 tected of the Solstein range are remnants from a primeval warmer period, and 

 were formerly distributed generally over the adjoining mountain ranges. These 

 cursory remarks should show that the accurate knowledge of the relation of heat 

 to individual species of plants may render important help in speculations about the 

 history of our vegetation. 



MEASUEES FOE PROTECTING GEOWING PLANTS FEOM LOSS OF HEAT. 



Since certain developments in plants have assigned to them the task of utilizing 

 external circumstances as far as possible so that heat may reach the growing 

 organs to the extent actually necessary, it is naturally to be expected that contri- 

 vances will not be wanting to protect them from an excess of heat, and also that 

 care will be taken that the heat once obtained is not again lost. It would not be 

 in harmony with what we know of the economy of vegetation that a plant exposed 

 to the sun should lose by radiation in the following night aU the heat which 

 it had gained during the day. It is known that growth Is carried on during 

 the night, and, indeed, that certain organs grow more in the night than in the 

 day, and in these an excessive loss of heat would be most disadvantageous. 



As a matter of fact arrangements exist for protecting plants from an excessive 

 loss of heat. These contrivances coincide in great part with those which regulate 

 transpiration, and have already been fully described in the discussion on that 

 subject, to which therefore we may refer. But those developments which claim a 

 particular interest as measures of protection against the danger of excessive losa 



