R. BRAITHVVAITE ONT THE HISTOLOPxY OF PLANTS. 293 



does the cambium lose its formative capacity at the close of this 

 period. In the resting period of temperate climates these cells 

 remain in the cambial condition, so that on the renewal of growth 

 in the whole extent, they may develope from new matter the 

 different elements of the vascular bundle. 



The wood and bast parts in this group attain their highest 

 development, and the three elements of each are far better defined 

 than in the vascular bundles of herbaceous plants, and also by 

 transverse section we plainly distinguish the primary medullary 

 rays, or Pith-hark rays, and the secondary medullary rays, or 

 Bundle rays, the former passing completely through, the latter, 

 only reaching a greater or less depth, according to the amount of 

 growth. 



A. The Wood imrt of the Vascular Bundle. — The Wood fibres, ox 

 fibre cells of the wood, originate from the elongated parenchym- 

 like cells of the cambium, and by subsequent extension acquire 

 the fibre-like form. In the perfect state they exhibit many 

 differences in the amount of thickening of their walls, and also 

 in the form of the secondary thickening layers. In the annual 

 ring we find this secondary thickening far more strongly developed 

 in the outer wood cells or autumn growth, than in the inner or 

 spring growth, and this is especially noticeable in Conifers. The 

 thickening layers are porose, with spiral thickening also in the 

 youngest, and the pores are always bordered, the border being 

 circular, and variable in width, and the pore canal usually slit-like 

 and oblique. The pores are open when they communicate with 

 each other or with vessels, but closed by a partition in those true 

 wood cells which later are used for storing up starch and other re- 

 serve material ((7/6/««^25, Hedera), In some cases a new formation 

 of cells takes place in them by tranverse partitions {Vitis, Hedera^ 

 Ruhus, &c.) and these constitute a transition to the next form. 



Wood Parenchyma. — The parenchymatous cells of the wood 

 are seen in nearly all woody dicotyledons, but are not found in 

 Berheris. Their walls are less strongly thickened, the unbordered 

 pores are always closed, and spiral thickening does not occur. 

 Their position varies ; sometimes they surround the vessels in 

 single layers or in groups, sometimes they form connected con- 

 centric bands, or sometimes they occur singly or scattered in 

 irregular groups. 



Wood Vessels. — The tube cells of the wood are formed by 

 solution of the transverse walls, thus constituting vessels as 



