C6 THE AMERICAN BOTANIST 



Duration of Leaves. — So common is the fall of the 

 leaf at the end of the growing season that we often seize upon 

 leaf duration as a convenient character for separating plants 

 into related groups. The coniferous trees are often separated 

 from the broad-leaved trees in this way and the former are 

 called evergreens though not all cone-bearers are evergreen. 

 The larch and the cypress cast their leaves in autumn as most 

 of our broad-leaved trees do. On the other hand such species 

 as the holly, the rhododendron and the magnolia as well as a 

 vast number of others in warm climates, do not cast their 

 leaves at the beginning of winter. Ultimately, however, all 

 leaves are thrown off though some may endure for many years 

 before the event occurs. The championship in matters of this 

 kind belongs, of course, to that curious west African gym- 

 nosperm, Welwitschia inirabilis, which produces but a single 

 pair of leaves and retains them through life, though it may 

 live for fifty years or more. Coming nearer home, we find 

 that the leaves of even common evergreens may attain a very 

 respectable age. Vinnie A. Pease, who has been investigating 

 this subject, reports in the American Journal of Botany that 

 some leaves of Abies grandis may live for 14 years, of Picea 

 Sitchensis 18 years, Pseudotsuga taxifolia 16 years, Taxus 

 bremfolia 23 years, and others in like proportions. These, 

 however, are the extremes. The average duration is seldom 

 more than half a dozen years, but even this is a ripe old age 

 when compared with the span of life of most leaves. Among 

 various shrubs studied, Arctostaphyllos uva-ursi in extreme 

 cases may hold its leaves for 6 years, Beiberis aquifoiia and B. 

 nervosa for 6 and 8 years respectively, Chimaphila unibellata 

 7 years, Gaultheria shallon 6 years, Kaluua polifalia ?> years, 

 Ledum Crocnlandieum 5 years, and / 'aecinium ovatum 7 years. 

 From various indications it is apparent that the plants which 

 grow the slowest retain their leaves the longest. 



