102 INTERNAL STRUCTURE [CHAP. 



daily on the outer side, with thick-walled prosenchymatous 

 cells, the outer portion of the trunk consequently becomes 

 much harder than the centre in woody Monocotyledons. 

 From the mode of development of the fibre-vascular 

 bundles, and the direction which they take in the stem, the 

 trunk of woody Monocotyledons does not usually increase 

 in diameter beyond a certain point, as we find in Palm- 

 Irees, which frequently have tall cylindrical stems as thick 

 at the top as at the base. These peculiarities led the older 

 botanists to call such stems endogenous, from a notion 

 that the younger bundles were those in the centre of the 

 stem, and that they pushed and compressed the older 

 bundles towards the outside. The only woody Mono- 

 cotyledon native in Britain is a shrub called Butcher's 

 Broom. In hot countries they are numerous, though 

 belonging to very few Families. 



12. But the structure of woody Dicotyledons, with 

 which, in our cool climate, we are most intimately con- 

 cerned, requires further examination. 



Take a cross-section of the stem of an Oak, several 

 years old. You find in the centre the remains of the 

 original cellular system of the stem, reduced to a very 

 narrow cord, and distinguished as the^/Y/;. When young 

 the pith served to contain stores of food for the nourish- 

 ment of the growing plant ; now it is dry and useless. 

 Surrounding the pith is the wood, forming the great mass 

 of the stem. A number of concentric rings are distin- 

 guishable in the wood, there being as many rings as 

 years that the stem has existed, one ring to a year ; so 

 taat by counting the rings from the pith to the bark you 

 may ascertain the age of the stem. The appearance of 

 rings, or annual zones, in the wood, arises simply from 

 the wood formed in summer and autumn being denser, 

 closer-grained, and with fewer vessels than that formed in 

 spring. 



Besides the concentric annual zones due to alternations 

 of summer work and winter rest, many timber-trees 

 exhibit in cross-section an inner and older portion of 

 the wood, differing in colour and often in other physical 

 characters, from the outer and younger wood. The inner 

 portion is usually more deeply coloured than the outer, 

 from which it may be separated by a definite line, nearly 



