Water-Conductivity of the Wood in Trees and Shrubs. 227 



localities in which it is never really subjected to drought. The wood is 

 capable of transmitting a limited amount of water, the leaves are fitted just 

 to utilise this amount with very little margin ; in other words, though the 

 plant requires little, it can hardly do with less. Furthermore, the roots 

 are able at all times to supply the small amount of water needed by the 

 relatively small, or else slowly transpiring, leaf surface, and this circumstance 

 is doubtless the prime factor both in determining the evergreen habit, and 

 also in limiting the situations in which the plants can flourish. Tlie subject 

 of the interrelation of root, stem, and leaf is a very complex one, and 

 obviously cannot be dealt with here. It must suffice to have indicated that 

 the stem structure may exert an influence in the whole problem which is by 

 no means negligible. 



It may be mentioned, also, that the creeping habit, combined with rooting 

 at the nodes, so often to be seen among herbaceous plants, is also commonly 

 (Jonneeted with a wood of defective conductivity. It is not improbable that 

 this correlation, which experience indicates very generally to exist between 

 the creeping habit on the one hand, and with badly conducting wood on the 

 other, is not merely a coincidence. Plants differ widely in the degree of 

 variability in this or that character, and creeping plants would certainly 

 repay experimental treatment from the point of view here under con- 

 sideration. 



Another plant of special interest in the present connection is the Butcher's 

 Broom (Buscus aculeatus). Attention must be called in the first place to the 

 fact that it belongs to the class of monocotyledons, and the structure of its 

 stem is unlike that of the plants hitherto considered. Owing to the circum- 

 stance that there is no clear demarcation between the wood and the cortex 

 without and the pith within, it is not easy to get a well-defined area for 

 measurement. The plan adopted was to include all the tissue within the 

 cortex as wood. This doubtless (as in the case of other monocotyledons) 

 means that the figure arrived at as representing the specific conductivity 

 is somewhat low. But it seems the best that can be done, and having 

 regard to the low absolute conductivity it is probable that it is not so far 

 from the correct figure as might be supposed. At all events the mono- 

 cotyledons themselves are fairly comparable inter se on such a basis. 



The Butcher's Broom is certainly one of the most striking examples of a 

 xerophyte, so far as habit goes, among all our native plants. And yet it avoids 

 habitats which might be regarded as consonant with its appearance. Like 

 the Daphnes, it requires very little water — far less, relatively speaking, than 

 they do. But it flourishes best in woodlands, and thrives in damp ground, 

 soon disappearing when really dry conditions set in, even though these may 



