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



endure for short periods of time. There is no question of invoking the facile 

 " explanation " that it is adapted to " physiological drought." Its associates 

 are mainly ordinary mesophytic, often almost hygrophytic, plants, and no 

 assumption of sour soil, or of unavailable or peaty water will satisfy the facts. 



Suggestions as to the supposed influence of " sour " soil have often been 

 advanced to account for the supposed xerophytic habit of rushes and sedges. 

 Nearer examination tends to show that such " explanations," however 

 attractive, are apt to be illusory. Neither rushes nor sedges are for the most 

 part at all adapted to endure real shortage of water.* It is true that some 

 of them use relatively little, but they are commonly extremely intolerant of 

 any diminution beyond their actual modest requirements. It is quite 

 possible to grow rushes in apparently dry garden soil, provided the water 

 table is not so far down that the capillary action of the soil is not at all times 

 able to supply the roots with the amount the plants normally utilise. The 

 small amount of water actually rendered available is to be attributed to the 

 defective conductive capacity of the wood, which in all such plants as Butcher's 

 Broom, Eushes, etc., so materially hampers a free supply. The same is true 

 of the aquatic Scirpus lacustris, with its green wand-like stems, and also of 

 the Equiseta. None of these plants are able to endure even short periods of 

 complete deprivation of water without wilting. 



It appears then that the xerophytic habit of the .plants just mentioned is 

 to be correlated very directly with the inherent structure oi: their own wood, 

 and not with any special feature in their external surroundings, or in 

 imperfect water absorption. 



An inspection of the figures given for Rtoscus will readily serve to make 

 clear the points alluded to above. Indeed so little water is transmitted per 

 \ hour, that it was necessary to continue each measurement for an hour, or 

 else to double the pressure for half-an-hour (which yields the same result) in 

 order to collect enough water for a reliable estimate to be made. 



The climbing plant E. (Semele) androgynus is strictly comparable with 

 a. aculeatus in respect of its stem, and as might be anticipated it is found to 

 possess a vastly superior vascular tissue, regarded from the point of view of 

 its conductivity. Like the climbing species of Asparagus, which have also 

 unquestionably sprung from xerophytic ancestors, it has largely increased its 

 green water-transpiring surface and it is freely branched. Its specific con- 

 ductivity varies within rather irregular limits, but the mean is about 40 in 



* In an interesting paper, by Dr. M. Delf, the point is made that the leaves of 

 halophj'tes ai-e not specially adapted to restrict transpiration. -It would be of interest 

 to examine the wood of some of these plants. See Delf, " The Meaning of Xerophily," 

 ' Journ. Ecol.,' III. 



