XX INTRODUCTION. 



the eastern half of the county. The Hog's Mill Eiver, flowing into 

 the Thames at Kingston^ and the Wandle at Wandsworth, only drain 

 the north-east corner. 



The streams of Middlesex require more minute description, as 

 the division of the county into districts for botanical purposes 

 is founded upon tbem. The Colne and its tributaries drain the 

 south-west of Hertfordshire. It rises on the north side of the 

 hills separating Middlesex from Herts, by several sources. A small 

 stream belonging to the Colne system commences at a point midway 

 between Elstree and Barnet, and enters Middlesex at Derham 

 Park; it then takes a north course, and draining the country lying 

 to the west of the Barnet and Hatfield Road, passes east of 

 South Mims, and re-enters Herts between that village and North 

 Minis; it has, however, no outlet; 'but the water is lost in 

 swallow-holes in the chalk,* that thus receives the drainage of 

 some twenty square miles of country' (^Whitaher, p. 20). Other 

 small streams originate in Middlesex from the north side of 

 Brockley Hill and on the east of Stanmore Heath, and flow into 

 the reservoir at Elstree,t a small portion of which is in Middle- 

 sex. The water of this reservoir flows north as a small stream 

 (and is by some considered the true Colne), and joins the Colne 

 at Colney Street in Herts. 



After uniting with several large streams (the Ver and the Gade 

 from Herts, and the Chess from Buckinghamshire), and having become 

 a navigable river, the Colne reaches Middlesex at Rickmansworth, 

 from which point it forms the county boundary for the remainder 

 of its course. It enters the Thames at a point between Egham and 

 Staines, by three branches. Just above Staines it sends off" another 

 branch, which passing Littleton flows into the Thames between 

 Sunbury and Walton Bridge. 



The chief tributary to the Colne during its course along the west 

 border of Middlesex is a stream draining Harrow Weald Common, 

 Pinner, and Ruislip, and joining it below Cowley. Ruislip Reservoir, 

 which receives a few rivulets from the north, is connected with this 

 stream. It occupies part of what was Ruislip Common. The water 

 is carried from it in an artificial stream along the course of the Cran 

 into the Paddington Canal near Southall. 



The Colne divides, during its course through the low ground called 



good example of the interdependence of the parts of a river system with reference to 

 their floras. 



* In Potterell's Park. See Fl. Herts, p. xxxii. 



t Also caUed Aldenham Eeservoir. 



