OLD AND YOUNG LEAVES. 347 



4. TKANSPIRATION DURING VARIOUS SEASONS OF THE 

 YEAR. TRANSPIRATION OF LIANES. 



Old and Young Leaves. — Fall of the Leaf. — Connection between the structure of the Vascular 



Tissues and Transpiration. 



OLD AND YOUNG LEAVES. 



The various regulators of transpiration, hitherto described, either persist in the 

 plant-organs in question throughout life, or only remain for a comparatively short 

 time. They are present throughout life in evergreen leaves, particularly in regions 

 where wet and dry seasons alternate during the year. In this case the plants 

 require powerful aids to transpiration in the rainy season, and in the dry season 

 abundance of protective measures against excessive loss of water. Evergreen leaves 

 cannot afford to dispense with either the promoting or inhibiting arrangements 

 after the expiration of the first year, because for several years they still have to 

 pass through both these seasons. It is otherwise with those leaves whose activity 

 only lasts for a single summer. These burst from the buds at the beginning of the 

 vegetative period, and then unfold, transpire and respire for a few months, pro- 

 ducing organic materials, and conduct them towards the places where they are 

 required. At the commencement of the drought, however, or on the appearance 

 of frost, they turn yellow and fade, are detached from the stems and branches 

 which bear them, and die. In leaves of this kind, an arrangement which is very 

 necessary during the first season may become superfluous later — it may even 

 become disadvantageous under changed external influences, and the leaf would then 

 be benefited by freeing itself entirely from the contrivance. It would often be 

 useful to the plant to substitute in the place of a protective contrivance, which is 

 only beneficial at the commencement of the vegetative period, another arrangement 

 fitted to the new and altered conditions. In the so-called deciduous leaves, i.e. 

 in those which throughout the year are only active in the summer, often only 

 for two months, it is a fact that an alternation of this kind may be regularly 

 observed in the mechanisms which govern transpiration. 



It will be noticed in a young foliage-leaf which has just pierced through the 

 ground, or in one which is still half-hidden between the cotyledons of a seedling, or 

 surrounded by the loosening scales of a winter bud, that the development of that 

 portion whose duty will later on be to transpire and assimilate, is very backward. 

 The leaf-veins are already very prominent, but the green tissue is in quite a rudi- 

 mentary condition. It is not only that the extent of the surface is very small, 

 but that the epidermis which covers it is not yet properly developed; the outer 

 walls of the epidermal cells are not yet fortified with a cuticle, and are consequently 

 neither water-tight nor impermeable to aqueous vapour. If exposed to sun or 

 wind, the green tissue would at once dry up. When the young foliage-leaf has 



