XVIU INTRODUCTION. 



and shelter that the hedges afford to the banks "below make the latter 

 peculiarly suitable for certain FUices, so that they attain on them unusual 

 size and luxuriance. In one respect the flora of Plymouth is poor ; it is 

 remarkably deficient in aquatic species. Owing to the hilly nature of the 

 country the area has very few still waters and sluggish streams ; hence it 

 is unsuitable for most water plants. Here, as elsewhere, we find surface 

 peculiarities distinctly influencing species distribution. 



It is now generally considered that the physical conditions of a soil — 

 its power of absorbing moisture and retaining heat — do more to determine 

 the species on it than do its chemical constituents. When we find in one 

 part of England plants limited to a particular soil which in others are 

 not so restricted, it seems reasonable to infer that it is the lithological, 

 not the chemical, nature of the soil which operates, through its affording 

 in tlie particular locality just the amount of moisture and heat required 

 by the species. 



CLIMATE. 



The mean temperature of the town of Plymouth is, according to Dr. 

 JMerrifield,* 51° 60'. This is no doubt higher than the average would be 

 for the whole area. Watson gives 52° as the assumed mean for Cormvall 

 on the southern coast-lme and 51 i° as that for South Devon. The mean 

 for some inland tracts within our boundary line would be certainly con- 

 siderably lower, owing to their elevation above sea-level, and also to the 

 fact that maritime localities are usually about one degree higher than 

 inland places under the same parallels of latitude. Watson assumes the 

 relation of temperature and altitude to be at the rate of one degree of 

 Fahrenheit's scale for each 100 yards of ascent.f At this calculation the 

 temperature at the summit of both Pen Beacon and Shell Top, the two 

 highest hills of our area, would be more than five degrees below that of 

 Plymouth through difference of altitude alone. As each of the six zones 

 into which Britam is divided in the Cyhele Britannica is estimated to 

 cover a range of about three degrees of mean annual temperature, it is 

 manifest that two are represented in our area ; the lowest or Infer- 

 agrarian, and the Midagrarian ; all the country up to 900 feet belonging 

 to the former, all above 900 feet to the latter. In the Flora of Middlesex 

 the mean temperature of Greenwich is stated to be 49° 4', as determined 

 from twenty years' observation ; so that the difference between it and 

 that of Plymouth, as determined by Dr. Merrifield, is 2° 56'. The authors 



* " The Meteorology and Climate of Plymouth." A paper read before the British 

 Medical Association, by John Merrifield, ll.d., ph. d., &c., Plymouth, 1871. The 

 references inserted above under " Climate" are to the pages of this work. 



t I)r. Merrifield notes that " The relation of temperature to altitude varies ^rith : 

 (a) the latitude ; (6) situation ; (<■ ) dampness or dryness of the air ; {d) calm or windy 

 weather ; (ej seasons of the year. The average is one degree for every 296 feet." 



