356 A. B. REXDLE ON THE USE OF MICROSCOPICAL CHARACTERS 



may cause disaster. But in timber from allied species, there may 

 be a close general resemblance associated with important specific 

 differences which are recognisable only by a microscopic study 

 of the minute structure. We are beginning to appreciate the 

 importance of an intensive study and a careful photo micrographic 

 record of the minute structure of the timber of species of economic 

 value. 



The forests of tropical Africa are rich in a great variety of hard 

 woods. Much of the so-called African mahogany which is put on 

 the market is the product of trees the genera and species of which 

 are not adequately known. The original Honduras mahogany is 

 the product of a Central American species, Swietenia mahogani, 

 the African woods belong to other genera of the same family, 

 Meliaceae, and there is need for a careful systematic study of their 

 species, as embodied in the characters of foliage and flower, in 

 association with the study of the minute structure of the wood. 

 The microscopist in association with the systematist can do work 

 of great importance. 



I have referred to the simplicity of the secondary wood in the 

 Gymnosperms as contrasted with the variety of elements of which 

 it is composed in woody Dicotyledons. The most striking dis- 

 tinction between these groups is the absence in the former of that 

 definite aggregation of sepals, petals, stamens and carpels which 

 we call a flower and the remarkable development and variety of 

 which is such a characteristic feature of the Dicotyledons. A small 

 family of the Gymnosperms, the Gnetaceae,is exceptional in having 

 a wood-structure similar to that of the Dicotyledons, and also an 

 arrangement which is somewhat suggestive of the flower of certain 

 Dicotyledons with a very simple type of flower. The question 

 arises whether this resemblance is an indication of relationship, 

 and an attempt has been made to trace the ancestry of the Dicoty- 

 ledons back through this group. But the three genera which are 

 associated in the family Gnetaceae are plants highly specialised 

 for life under somewhat peculiar conditions — one is a tropical 

 woody climber, the others are adapted for growth in desert areas — 

 and one hesitates to draw far-reaching conclusions on relationship 

 from such specially adapted organisms. We bear in mind the 

 remarkable differences in habit and vegetative structure of species 

 even in the same genus where they are adapted to different 

 conditions of life. 



