96 



THE PHYSIOLOGY OF PLANTS 



shoot of the vine. It will then be noticed that only a little 

 of the soft green tissue will be removed, and then there will be 

 laid bare a number of firm whitish bundles of fibres running 

 in longitudinal direction. These are the hard bast fibres or 

 stereom. They consist of long spindle-shaped and thick- 

 walled cells, usually without contents, but very flexible. They 

 are united into strands running on the cortex, and form the 

 material which we use for tying up plants. These bundles of 

 hard bast run parallel to the vessels of the wood, and under 

 the microscope it will be seen that in a young branch one such 

 zone of bast lying in the cortex corresponds to each portion of 



wood lying between two 

 primary medullary rays. 

 Such a group of hard 

 bast cells has often a 

 semilunar appearance, the 

 convex side turned towards 

 the outside of the stem. 

 Protected on the outside 

 by this group of hard bast 

 cells, we find a number of 

 strands of delicate cells, 

 which either have the 

 Fig. lo.— PART OF A Transverse Section THROUGH same appearance as the 



cambium cells {camhi- 



A Vascular Bundle of Lagenaria vulgaris. 



m widely reticulate, ?! closely reticulate sieve-plate. 



Each is the horizontal transverse wall of a sieve- JOrm), Or WblCfi represent 

 tube(«/^.rDEBARY). ^^^^^^ ^j^-^j^ j^^^^ ^^^^ 



formed by the fusion of a number of superposed cells. These 

 tubes are characterised by the fact that the transverse walls 

 which separated the original cells have not entirely dis- 

 appeared (as in the wood vessels), but are perforated after 

 the manner of a sieve. These tubes have therefore been 

 termed sieve-tubes, and together with the eambiform cells they 

 constitute the soft bast, as distinguished from the above-men- 

 tioned hard bast. It is along this soft bast tissue that the 

 organic plastic material (assimilated material) which has 

 been formed in the leaves passes to the regions where it is 

 required. 



Fig. I o represents a more delicate and a coarser sieve-plate. 



