ON THE EFFECTS OF EXCESSIVE DROUGHT UPON PLANTS. 505 



ON THE EFFECTS OF EXCESBIVE DEOUGHT UPON 

 PLANTS; OE THE OEIGIN OF XEEOPHYTES. 



By the Eev. Peofessor G. Henslow, M.A., V.M.H. 



[Bead October 24, 1911; Dr. A. B. Rendle, F.R.S., in the Chair.] 



In my first lecture this year I illustrated the effects of an excess of 

 water upon plants. In the second it was shown that the Monocoty- 

 ledons were descended from aquatic Dicotyledons. In the present 

 lecture I propose considering how plants adapt themselves to exces- 

 sive drought. 



Roots. — If sufficient water be present in the soil or sand, as in 

 deserts, but at a considerable depth, the root-tip is stimulated by the 

 slight dampness arising from below and continues to grow till it 

 reaches it. Hence small annuals in the desert near Cairo often have 

 roots ,a foot or more long,,- while those of some old specimens of 

 Colocynth have been traced to very many feet in length.* In allusion 

 to this power of growth in " search of water, " Dr. Lindley observes : — 

 " Plants, although not locomotive like animals, do perpetually shift 

 their mouths in search of fresh pasturage, although their bodies remain 

 stationary. As an example, in a garden at Tumham Green a Populus 

 monilifera (Canadian Poplar) was found to have sent a root thirty 

 feet horizontally, including its dip, beneath the foundations of a wall, 

 and then to have passed into an old well to the depth of eighteen feet, 

 having then broken up into a mass of fibres so finely divided as to 

 resemble yarn."f Similarly, a turnip root penetrated a field drain 

 I and grew to upwards of six feet in length. 



I Water-storage Tissues. — These occur in all parts of plants in 

 I deserts, and even the roots are not exempt. Thus three species of 

 ' Er odium bear tuber-like swellings on their roots for this purpose. 



As root-hairs are generally absent on roots living in water, so, 

 j on the other hand, are they abundant when moisture is scarce. The 

 [ late Dr. M. T. Masters found that when mustard was sown in 

 rammed clay, the radicles penetrated it to reach the side of the pot. 

 Having done so, the roots produced an abundance of root-hairs." 

 This he attributed to the presence of a thin film of moisture. 



Stems. — A hard wood is characteristic of dry regions. Thus all 

 the trees of Cape Colony are remarkable for their solidity. Experi- 

 ments by P. Eberhardt showed that the same species of woody plants 

 grown in very dry air, under normal conditions, and in very moist air 

 developed different amounts of woody tissue; much wood being 

 developed under the dry conditions and very little under the moist. 



* For other illustrations see my Origin of Plant Structures, p. 52. 

 t Theory and Practice of Horticulture, p. 19. 



