384 MR. G. F. SCOTT ELLIOT ON THE EFFECT OF 
Extremely thick epidermis, moreover, characterizes species 
which are naturally found in particularly exposed habitats 
(compare Volkens, Schwendener, Tschirch, Giltay, Heinricher, 
Fleischer, Johow, and Schimper). 
Now it seems to me that what is called in Germany “the 
inner causes of growth” may, without wild speculation, be looked 
upon as an actual force within the leaf, which tends to elongate 
it in a direction parallel to the midrib. At any rate, they may 
be supposed to act in this way. 
Now if, as is really proved, the epidermis becomes tougher ın 
an exposed habitat, it seems natural enough that the greater 
resistance to this elongating force would make the leaf tend to 
become broader proportionately, as well as thicker (cf. Hoffman 
and Seholtz). 
This force, however, has not been proved to exist. 
It struck me as being an interesting point to see whether the 
petals are also influenced by exposure and tend to become longer 
and narrower when grown in sheltered places. 
I therefore made a series of measurements on the length of 
the corolla-tube in Vinca rosea. 
One set (Form A) were taken from plants sheltered by grass 
and other plants. The others (Form B) from particularly exposed 
specimens. 
I found :— 
Average length corolla-tube in Å .....- "982 
» » » Biss ‘943 
So that the difference is not marked enough to judge from. 
The effect of the soil is, however, much more easily visible. 
Thus, in the experimental garden before alluded to, I measured 
in Tropæolum the length along the spur from the tip of the 
sepal to the tip of the spur, and the lengths and breadths of the 
anterior petal. I found :— 
Peat. ene) Sand. Lime. Mould. 
Length of spur ........ ...... 1522 | 1875 | 1964 | 1975 2182 
| Length anterior petal ...... 1-05 1:375 | 1463 1359 | 15 
| | | 2 
| Breadth anterior petal...... ? da 3 od m | x 
| 
Ratio (approximate)......... ? PR | 1e | 15 | 16 | 
| | | 
