29G R. BRAITHWAITE ON THE HISTOLOGY OF PLANTS. 



part of the Vascular bundle, and only moderate in tlie wood ; in 

 the bast part the cellulose case of the intermediate tissue remains 

 unlignified, but is generally lignified in the wood part, and only in 

 rare instances we find portions of the Primary rays changed into 

 hard, strongly thickened woody tissue {Fagus, Betula) ; closed 

 pores alone are found in the thickening layers. 



The contents during the resting period are always starch 

 granules, but at the growing period these are few and floating 

 in other formative material. The size of the cells and number 

 of rows in each Bundle-ray vary in different species of trees, and 

 are of importance in studying the structure of their wood. 



Vascular Bundles of Cycads and Conifers. 



These agree in their mode of origin with those of Dicotyledons, 

 but differ in tlie occurrence of single forms oi cells, and their 

 more composite arrangement, so that the wood and bast parts may 

 be at once distinguished from those of other trees. 



The Wood part. — Wood fibres. The fibre cells form the 

 principal mass of the wood, and always show distinct large 

 bordered pores ; the border being circular or sometimes elliptic, 

 but very rarely angular as in the root of Ai aucaria and wood of 

 Cycas ; the pore canal is also circular, but in Cycas slit-like. 

 Besides the porose, spiral thickening also occurs ; the secondary 

 thickening does not show lamination, but in the wood of our 

 Conifers we find that in the outer or autumn part of the annual 

 ring the cells are so strongly thickened that scarce any cavity is 

 left, but in the inner or spring part there is only slight thickening. 



Wood Parenchyma. — This is the same as in other trees, but is 

 less generally distributed ; in Cycads it is present as single cells 

 or in small groups, but in Araucaria altogether wanting ; mAhies it 

 forms scattered cells in the wood substance, which at an older stage 

 become turpentine canals ; in Pinus and Larix it is not scattered, 

 bat fornjs the circumference of the turpentine canals. 



Wood Vessels. — The tube cells are never wanting in the primitive 

 wood bundle of Cycadece and Pines, and appear with spiral or 

 netted thickening. They are absent from the older wood of Cycads, 

 and also from" Abietinew, CvpressinecB, Araucaria, Taxinece, and 

 Fodocarpece. In Gnetacece only true vessels with porose thicken- 

 ing occur through the whole wood substance. 



The Bast part. — Bast fibres. The fibre cells have a similar 

 structure to that of other woods, and appear in Cycadece in the 



