226 Prof. J. B. Farmer. On the Quantitative Differences in the 



occur in the wood of this plant. Pernettya, on the other hand, gave an 

 average (six observations) of 5 with scarcely any deviation. 



Escallonia macrantha, as might be expected from its habit, is higher up in 

 the scale with an average of 24, and 6 of the 7 stems investigated were 

 inside the range of +2. The deciduous Azalea mollis averaged 30-5 with a 

 range in 8 of the 9 stems examined of + 3-5. 



The species of the genus Daphne are of somewhat special interest as regards 

 their relations to water. They are slow-growing plants, and include both 

 evergreen and deciduous species. Among the latter the common D. Mezereum 

 shows a near approach to evergreen habit, and lacks the abundant foliage of 

 ordinary -deciduous shrubs. In short it belongs to the class of plants often 

 terrned xerophytes, because they are supposed to be adapted to a dry environ- 

 ment, and yet it is a matter of common experience that the plant does not 

 really thrive under xerophytic conditions, and this is even more true of 

 actual evergreen species such as D. Blagayana, D. Laureola, etc. It is 

 true that these will live in soil that is well drained, but they are by no 

 means tolerant of drought. D. Laureola is commonly a woodland plant, 

 and it grows best by the side of water. D. Blagayana, a plant that is some- 

 times difficult to grow, often refuses to succeed when exposed to sun and 

 drought in the drier climate of the Home Counties. It requires to have its 

 stems layered and to be earthed up with stones and leaf mould. If this is 

 not done the shoots are apt to die back by reason of the inadequate supply 

 of water which reaches the terminal cluster of rather large leaves. When 

 earthed up, however, adventitious roots spring from the stem and make good 

 the deficient water supply. For not only is the supply more copious, but it 

 is also more immediately at the service of the leaves by reason of the shorter 

 distance to be traversed through the wood. 



All the species I have examined possess a wood of very low conductivity. 

 D. Mezereum, as might be expected, is the highest, at about 7. 



The low conductivity is clearly connected with the slow growth, and, 

 like the latter feature, is a strongly marked and inherent character of the 

 whole group. Nevertheless, the plants can hardly be, with propriety, termed 

 xerophytes. The rather large leaves of the evergreen species are badly fitted 

 to resist transpiration. They are reduced in number, it is true, but those 

 present actually transpire rather freely. It is unquestionable that the 

 limiting effect of the wood materially influences the reduced leaf area, 

 although questions as to root efficiency are also bound up with the matter, 

 at any rate in terrestrial plants. Those leaves that are present require 

 supplies enabling them to transpire somewhat freely. Hence it becomes 

 intelligible why a plant apparently xerophytic may yet be restricted to 



