CO-OPERATION AND THE ORIGIN OF FLOWERS. 15 
Our 
posure.* I have noticed this especially at Sligo Bay and on 
rocks near the seashore. 
On the Pyrenees, a tree of Juniperus Nana may be 16 
millimetres in height though 57 years of age.” 
This dwarfing is the result of suppression of internodes ; 
elongation from the bud stage 1s prevented. Kerner von 
Marilaun was, in fact, able to produce internodes on the 
stems of typical rosette plants, such as Shepherd’s Purse, 
Dandelion, and Houseleek, by growing them in a moist and 
humid atmosphere. 
The physiological explanation of this effect of a dry, 
climate or of wind is quite simple. 
It has been shown by direct measurements that the 
thickness of the epidermis outer wall varies directly with 
the transpiration. The leaves of the Bracken, for instance, 
show well-marked differences in exposed and in sheltered 
places.25 Exposed plants in general have thicker cell walls 
as compared with the flora of sheltered places. + 
Such thicker and stronger cell walls will tend to prevent 
elongation. Moreover, transpiration itself depends on the 
difference between the pressure of water vapour in the inter- 
cellular spaces within the plant and that of the outside air. 
Wind removes at once any vapour leaving the stomata, and 
may increase transpiration five times, as has been proved by 
direct experiment.” 
Transpiration in excess not only increases the resistance 
to expansion in growth, but decreases the water pressure, to 
which elongation is due. 
And so the flower ancestor would be under a continual 
temptation to refrain from producing internodes between its 
sepals and petals. It would thus form a circle of sepals and 
a circle of petals. We would expect that the numbers of 
each of these would be either three or five (arising from 
one-third or two-fifths phyllotaxis), or two and four if the 
* Exact and detailed observations are again wanted (seventh 
series). 
t+ There are but few observations on this interesting point in 
‘Great Britain (eighth series), | 
