116 PRIZE ESSAY: 



on the upper side alone ; but they exist equally on both surfaces 

 of erect leaves, as in the lily tribe and grasses, (*) 



204 (). Cellular tissue ( 2 ) exists in all plants, and composes a 

 large portion of turnips, carrots and other fleshy roots. It con- 

 stitutes the pith and outer bark of trees, and the central part of 

 rushes. The little cells of which this tissue is composed vary in 

 size. They are found from 10 1 00 th to y^th part of an inch in 

 diameter. The general average diameter is from ^-th to x^jth 

 of a line, and that of the cellular spores of fungi -go^th of a line 

 or ^^j-Qth of an inch in diameter. 



205. Vapour of water passes from the surface of plants in two 

 ways, either by evaporation or exhalation. Evaporation from 

 the surface of plants is dependent upon the moisture in their 

 tissues, the temperature of the air and the dew point. When 

 air is saturated with moisture, or in other words, when the dew 

 point is the same as the temperature of the air, evaporation from 

 the surface of plants ceases. It is entirely independent of vitality. 

 Exhalation is a function of the plant ; is altogether dependent 

 upon vitality, and bears a strict relation to the number of stomata 

 on the plant. 



206. Exhalation is greater in summer than in autumn, and is 

 much less active during the winter than at other periods of the 

 year. A laurel parts with as much fluid in two days in summer, 

 as during two months in winter. < 3 ) Hales found that a common 

 sunflower transpired on an average 20 oz. a day. The weight of 

 the plant was 3fts., its height 3 feet, and the surface of its 

 leaves 5,816 square inches. On one warm day it exhaled as 

 much as 30 oz. of fluid ; on a warm dry night 3 oz. ; when the 

 dew was sensible, though slight, it neither lost nor gained, and 

 by heavy rain or dew it gained 2 or 3 oz. (*) 



(1) Carpenter. (2) Called also Parenchyma. (3) Guettard, quoted by Carpenter. 

 (4) Quoted by Carpenter, Prin. Cornp. Physiology, 



