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YORKSHIRE NATURALISTS UNION. 



probable, in some cases at least, viz., that broken fragments of 

 the stem (which readily take root at every joining) may be 

 conveyed from one piece of water to another entangled in the 

 plumage or feet of water birds. 



Another problem is — What is the explanation of the occasional 

 reappearance of mountain plants, especially cryptogams, on barren 

 sandy heaths in the plains ? The question was raised by the 

 discovery by Mr. F. A. Lees of a well known Arctic and Alpine 

 lichen, the so-called "Iceland Moss" {Cetraria Islandica) in pine 

 woods on the greensand near Market Rasen, in North Lincoln- 

 shire, since found also in heathy pine woods on alluvial sand at 

 Holme-on-Spalding-Moor in the East Riding, and said by the 

 Rev. W. A. Leighton to occur at King's Lynn. Other examples of 

 a similar nature are Lycopodium davatum, inundatwn and alpinum, 

 formerly found by the Rev. W. Fowler on Crossby Warren, North 

 Lincoln; Racomitrium canescens on Santon Warren, North Lincoln, 

 and on Strensall Common near York; Cetraria aciileata, Platysma 

 glaucum and Cladonia uncialis on Riccall Common; Empetrmn 

 nigricm on Goole Moor, &c. Two explanations may be suggested : 

 such species may be either intruders introduced by some means 

 into the stations which they occupy, or stragglers left behind in 

 the general retreat and able to hold their ground under favorable 

 circumstances against the invading host of new species brought 

 in by a change in physical or climatic conditions. It was thought 

 by the writers that, in the above instances, the second explanation 

 was probably the correct one, and that the plants in question 

 were remnants of a boreal fauna left behind v/hen the ice and 

 cold of the glacial period retreated into more northern regions. 

 The circumstances favorable to such plants possessed in common 

 by sandy heaths and mountains appear to be plenty of fresh air, 

 an atmosphere free from smoke and other impurities derived from 

 the presence of man, humidity, and above all the absence of undue 

 competition of other plants, especially of the herbage plants of 

 pastures, and of the weeds of cultivation; this freedom from com- 

 petition being secured in one case by the cold and elevation, in 

 the other by the barrenness of the soil. 



Trans. Y,N.U., 1878. Series E 



