Moss : Changes in the Halifax Flora. 



171 



sagittifolia L., Potamogeton nutans L. , P. crispus L., P. obtusi- 

 folius M.&K., P. pusillus L., P. pectinatus L., Zannichellia 

 palustris L. , and Eleocharis acicularis R.Br. 



A few moorland aquatics were without doubt present before 

 Bolton's time, such as Potamogeton polygonifolius Pour, and 

 Ranunculus Lenormandi F. Sch. ; and perhaps a few lowland 

 aquatics too, such as Alisma plantago-aquatica L., though these 

 are unrecorded by him. 



Judging- by the profuse aquatic vegetation in the Halifax canals 

 and mill-dams, lowland aquatic types seem to be less affected 

 by contamination of the medium in which they live than any 

 other class of plants. This generalisation does not apply to 

 those aquatics — few in number — which inhabit swift-flowing 

 streams. 



Thus far it may be seen that the changes noted in the 

 Halifax flora may all be ascribed to artificial causes, which may 

 be summarised as follows : — 



1. Encroachment of towns and villages ; pollution by smoke, 

 sewage, etc. ; the conversion of heaths into parks. 



2. The drainage of the moors to supply the reservoirs; 

 the increase of tillage on the moor edges ; the systematic 

 burning of the moors to supply grouse with cover, and the 

 more casual burning by irresponsible people. 



3. The depredations of trippers, herbalists, gardeners, and 

 naturalists. 



It has also been shown that those portions of the parish 

 which have been least molested by man exhibit very little 

 change at all ; and that plants which have been observed only 

 recently are mostly aquatics, and are consequent on the opening 

 of the canal. 



There are other artificial causes of change of plants, which 

 do not appear to have been obtrusively operative in Halifax 

 daring the last 125 years. 



At Woodlesford, near Leeds, the formation of hollows in the 

 surface of the land due to the sinking of the roofs of mines, has 

 resulted in the formation of pools of some considerable area ; 

 and a host of aquatics has appeared, where before the vegetation 

 was quite terrestrial. 



The felling of timber would cause a change from shade types 

 such as Scilla nutans to plants more xerophilous in habit. hi 

 Halifax, however, it is not often that a whole wood is cut down 

 at once, but only trees of a certain thickness, so that the general 

 conditions of our woodlands remain unaltered. 



iqoo J 11 iu- 1. 



