V. 



The Falling of Leaves. 



IT is a matter of common observation that most of the trees in 

 temperate climates Hke our own are deciduous, that is, shed their 

 leaves at the approach of the unfavourable season to break out anew 

 when it is past. The roughest examination of a leaf shows tlie reason ; 

 a delicate structure spread out to get as much air and sunlight as 

 possible, and containing a high percentage of water, would fare badly 

 in the frosts of winter. The plant, indeed, has often to resort to 

 devices for protection against the cold of even a summer night, as 

 witness the sleep movements of leaves ; the false Acacia {Robinia pseud- 

 acacia), for instance, dropping its leaflets in to a vertical position, instead 

 of, as in the day-time, spreading them almost horizontally to the sky. 

 An early snow-storm shows the foliage to be actually a source of danger 

 to the tree in winter, the great weight of snow, kept from falling by 

 the leaves, often snapping huge branches, or bearing down the whole 

 tree. Where leaves persist through the winter, as in evergreens, they 

 are specially protected by a thick cuticle, and often much reduced in 

 size, as in Pine " needles." 



Common though the occurrence is, it is only in recent years that 

 we have had anything beyond vague guesses as to the way in which 

 it is brought about. Various suggestions were made from time to time, 

 but they were not based on a careful investigation of the parts con- 

 cerned, and were, therefore, generally valueless. Thus Duhamel (i), in 

 his " Physique des Arbres" (1758), states that a layer of tissue between 

 the leaf and stem remains in a herbaceous condition, and cannot stand 

 the cold of winter ; and this being the case, evergreen trees keep their 

 leaves by reason of the point of insertion acquiring a maturity suffi- 

 cient to resist the severe weather. He admits, however, that this 

 explanation is not altogether satisfactory, for it does not explain why 

 leaves which only appeared at the beginning of the autunm, resist 

 the cold better than those developed in spring, whereas the latter 

 ought certainly to be the less herbaceous, and accordingly suggests as 

 a second cause that the leaves stop growing on account of their 

 strong evaporation of water, while the stem continues to grow in 

 thickness, thereby setting up a mechanical strain and tearing the 

 fibres connecting the two members. 



According to Mustel (2) (1781), the leaves cease to evaporate in 



