PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 307 



coast district on a comparatively narrow strip of land; while in the interior of Istria 

 the waste dry terraces of the Tschitscherboden (which might be compared with the 

 arid plains of the Cape) show no trace of a heath vegetation. 



Why the plants with evergreen leaves which grow in the far north, on the 

 heights of the Alps, on the Baltic lowlands, on the shores of the Atlantic Ocean, on 

 the borders of the Mediterranean basin, and at the Cape of Good Hope are not all 

 of the same species, is a question which cannot be answered here; yet it seems 

 proper to point out that all plants furnished with evergreen rolled leaves, whose 

 year's work is stopped by dryness, would freeze in countries where the earth in 

 winter is covered with snow, i.e. the molecular structure of their protoplasm would 

 be entirely altered by the frost, which would kill it; while the protoplasm of the 

 analogous northern forms would suffer no harm from the cold. It is well worthy of 

 remark in this connection that some of the last-mentioned plants have an extra- 

 ordinarily wide distribution; that they may actually be found, quite similar in 

 appearance, in the bleak north, and in the southern districts, if only those conditions 

 of moisture which we have shown to account for the form of the leaves obtain in 

 the places mentioned. Thus the Irish Heath (Dabeoeia polifolia) may be found 

 along the Atlantic coast as far as Portugal, and the common Ling (Calluna vulgaris) 

 grows just as well at a height of 2450 metres above the sea beside the glaciers of 

 the (Etzthal in the Central Alps, as further south on the Abazzia, surrounded by 

 laurel groves on the sea-coast of Istria. 



3. PREVENTION OF EXCESSIVE TRANSPIRATION. 



Protective arrangements on the Epidermis. — Form and Position of Transpiring Leaves and 



Branches. 



PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 



The relation of the form of the evergreen rolled leaf to transpiration is anything 

 but exhausted in the foregoing account. The part played by this form of leaf, in 

 particular during the dry season of the year, yet remains to be discussed. If it is 

 necessary during the wet period that transpiration should be increased as much as 

 possible, and that everything which might restrict the exhalation of aqueous vapour 

 from the stomata should be kej)t away, it is also of importance that on the appear- 

 ance of the dry season the equilibrium between the water taken from the soil and 

 the water excreted by the leaves should not be destroyed, and that an excessive 

 evaporation from the portions of the plant above ground should be hindered. New 

 seasons bring new problems to be solved. At the time when the water-current 

 begins to ascend from the soil saturated by the winter rains, we have an aid to 

 transpiration; later on, in the dry period, we have a protection against the dangers 

 which might attend excessive evaporation. It is certainly of great interest to see 



