GLASGOW SOCIETY OF FIELD NATURALISTS. 201 



favourably affected by man's influence directly, or through his 

 domestic animals indirectly, as in the case of Trifolium pratense. 



I. Under the first heading in the table, we may take the well- 

 known instance of the house-leek (Sempervivum tedorum), so 

 called from its growing almost exclusively on thatched roofs. 

 This plant has thick succulent leaves, and such leaves have com- 

 paratively few stomati on their surface, so that their power of 

 exhaling watery vapour is limited. For this reason many 

 succulent leaved plants succumb to a damp climate (" Origin of 

 Species "), but are specially fitted for a dry one. Dr. Livingstone 

 has recorded that the Kalahari desert, when first taken possession 

 of by the Boers, was covered with a coating of grass, but that 

 subsequent droughts, proving too much for this form of vegetation, 

 it has disappeared, and its place has been taken by a crop of 

 Mesembryanthemums, and that these are now cultivated largely 

 by the Boers, as fodder for their sheep and goats, in place of grass. 

 These Mesembryanthemums or ice-plants resemble the house-leek 

 in possessing thick succulent leaves and few stomata. On a thatched 

 roof the latter probably finds the conditions of its primitive home 

 pretty closely imitated : thus, it is exposed to the full force of 

 the sun's rays, and excess of rain is rapidly drained off. For this 

 reason the house-leek flourishes, whilst the grass upon the house- 

 top, to quote the Psalmist, " withereth afore it groweth up." 



II. Tap-roots and stem-tubers, there can be little doubt, are 

 organs, the primary intention of which is to contain a supply of 

 water to enable the plant to live through a long rainless period. 

 That this is so, is shown by the large proportion of plants which 

 bear such roots and tubers, occurring in the periodically parched 

 districts of Africa visited by Livingstone and Schweinfurth, as 

 well as by the great size attained by such structures in these 

 regions of periodic drought ("Heart of Africa"). Many shore 

 plants possess roots capable of succulent development under the 

 influence of stimuli ; indeed, our carrots, turnips, beets, and even 

 the potato itself, were originally shore plants. The possession of 

 these roots would enable the plant to take a supply of water, when 

 that was to be had, before the rain had all filtered off through the 

 gravel. Under cultivation, from the large percentage of water 

 these roots contain (turnips 91 %); and from the great perfection 



