THE SPKUCE FIR 253 



cells at the ends of the finer bundles in dicotyledonous 

 leaves. It is supposed that they collect the proteid 

 substances formed in the mesophyll, and transfer them 

 to the phloem for transport into the stem. 



The conjunctive tissue further contains a strand of 

 thick-walled fibres placed just outside the phloem 

 (see Fig. 101). We see, then, that the tissue of the 

 cylinder is altogether rather complicated, for besides 

 the vascular bundle and the ordinary parenchyma- 

 tous cells, in which .starch is formed, it contains the 

 transfusion- tracheides, the albuminous cells, and the 

 sclerenchymatous fibres. 



The leaves remain on the tree for three or four 

 years. During this time a certain amount of new 

 tissue, chiefly phloem, is formed by the cambium. 

 Secondary growth in a leaf is not so very common, 

 and is limited to leaves which last for a long time. 



The leaf is constricted at the base, so that the 

 mesophyll here comes to an end. Only the cylinder 

 extends through into the stem. Hence all the food- 

 material which goes to and fro between leaf and stem 

 must pass through the tissues of the cylinder, which 

 thus forms part of the conducting system of the plant. 



The scale-leaves which protect the buds have a 

 very simple structure compared with the foliage- 

 leaves, for they have no assimilating work to do. They 

 often contain no vascular bundle at all; two resin 

 canals, however, are present in the mesophyll. The 

 epidermis on the outer surface of the scale has an 

 extremely thick cuticularised external wall, and this 

 is no doubt the most important part of the whole 



