452 BOTANICAL GAZETTE [DECEMBER 
eaves toes Leaves Leave — aromatic 
summer withering paraheli- | sulcate or hairy or with 
deciduous otropic revolute PPR PeG Ser — 
Artemisia arborescens..... x x x 
Artemisia variabilis........ x Bie a x 
Helichrysum rupestre...... x x x x 
TGla VincOsA ok a * a ae rae x 
Medicago arborea......... x i x Ae os 
Matthiola rupestris........ ie me 2 x x 
No one of the nine cliff species enumerated is without some xerophytic 
characteristics, but it is noteworthy that only two of the non-succulent 
species have ample and obvious protection from injury by drought. The | 
Spartium, leafless during the drier months and with thick wax-coated 
epidermis and sunken stomata, endures months of rainless heat without 
injury. The Medicago (figs. 2, 3)* has a far less xerophytic aspect, but it 
flourishes not only on the nearly vertical surfaces of tufa cliffs, but also on 
the bare tops of ancient Roman walls and pillars. It is, indeed, the char- 
acteristic shrub of the meager flora of these latter localities, often at a height 
of 10-20™ above the ground and far beyond the reach of any moisture 
from the soil. Its roots are short and scanty, and the plant contains no 
mechanism for special storage of water, but the complete shedding of the 
leaves in early summer renders the shrub secure against fatal desiccation 
afterward. Unfortunately I was not able to make as many determinations 
of the rate of transpiration of M. arborea under various conditions, as 
would have been desirable, so the following values are far from exact; 
still they may serve to explain the tenacity of life of the species. The 
total transpiration per hour at 30° C. of a twig 8°™ long was about as 
follows: 
May i kaiytwiginwater. 6 5 =... oy 80™ 
ey 14 tealy twig in water <9. «Se 
pune47 leafy twig in water. 6 ws. 0 
September 14, leafless twig,notin water ... . 5 
In the first case above given, the leaves were in the height of their 
“activity; May 14 they looked as green and fresh as at first, but had lost 
a little of their power to transpire. By June 17 all the leaves had turned 
_ Yellowish and had taken up permanently their paraheliotropic position. 
On September 14 the twigs were (and had long been) entirely leafless and 
appeared rather destitute of moisture. The figures speak for themselves, 
and the final rate of transpiration, less than 3 per cent. of the maximum, 
« For these figures the author is indebted to Mrs. HERBERT S. JENNINGS. 
