16 PROPERTIES OF WOOD. 



wood. This question will be discussed fully under the head- 

 ing, "Weight of wood." Only when the whole annual zone 

 consists of merely two cells, as in very suppressed stems, one 

 may be termed the early cell and the other the late cell, 

 and these, naturally, can be seen only through a microscope. 



The commencement of the non-growing season is charac- 

 terised by the close of the annual ring, by the suspension of 

 cell-division in the cambium, by the emptying of the plasmic 

 contents of all wood-cells except those of the parenchyma ; 

 deciduous foliage loses its green colour and falls several weeks 

 after the completion of the annual zone of wood. The shorter 

 the period of .rest, the less distinct are the signs of the 

 annual rings in broadleaved wood. In sub-tropical, evergreen 

 oaks there are no ring-pores ; woody species in the tropics, 

 where there is no season of rest for vegetation, have a com- 

 plete period of cessation in the growth of wood whenever the 

 tree in question loses its leaves, even if this should be but for 

 a short period; should leaf-fall recur annually at stated times, 

 for instance during the dry season, annual zones of wood are 

 formed, as in the wood of teak, of several tropical leguminous 

 trees, etc. In most tropical woods, traces of zones resembling 

 annual rings may be seen on a transverse section, but they 

 cannot be considered as such, any more than similar formations 

 in the wide late wood of pines, especially of the most southern 

 species. 



The formation and structure of the wood of monocotyle- 

 donous plants, palms and bamboos, differs from that of broad- 

 leaved trees and conifers. In palm-wood (Fig. 27) the 

 commencement of wood-formation is also in the bud ; 

 isolated vascular bundles appear in a pith -like primary 

 tissue, as continuations of the vascular bundles (ribs or 

 nerves) in the leaves, which spring from the single, 

 unbranched, stem. These bundles are not, however, 

 arranged in a circle, do not become united by a cambium, 

 and are therefore unable to form a cambium for future dia- 

 metric growth. In the transverse section of a palm-stem, the 

 thickest vascular bundles are those that are innermost; out- 

 wardly they increase in number but are smaller, so that the 

 outermost layer of a palm-stem is the hardest ; the thin outer 



