THE STEM loi 



But these tissues of the medullary ray, where they separate 

 two cambium layers, assume later on the same cambiform char- 

 acter, and thus form a bridge (Fig. 1 2, G, ic), which connects 

 two adjoining pieces of cambium, and in this way a continuous 

 cambium ring is ultimately formed (Fig. 1 2, C, fc and ic). 



What appears in transverse section as a ring is, however, 

 in reality a cylinder of cells which reaches through the whole 

 length of the axis. The stem or branch, therefore, will now 

 appear to be a column, the centre of which is occupied by a 

 narrow but solid cylinder of pith. Around this we find a 

 hollow cylinder of wood, which is covered in by a mantle of 

 cambial tissue, bounded on the outside by the bast, still more 

 externally by the cortex. 



The view, therefore, which is still to be found in some of 

 the older books that the cambium is a fluid, the so-called 

 formative fluid, is erroneous. Cambium is not a formative 

 fluid, but a formative tissue. Such formative tissues, which 

 are composed of delicate closely-packed cells, full of protoplasm 

 and plastic substances, and which have retained their powers 

 of division, are termed meristematie tissues. Cambium is there- 

 fore a meristem. 



So as to understand more clearly the position and the nature 

 of the cambium, let us examine the transverse section of a 

 one-year old shoot of the Laburnum (Cytisus Laburnum) which 

 was cut in the month of May (Fig. 13). 



The portion of the section figured contains only the essen- 

 tial tissues. It does not include the cortex, which would lie 

 on the farther side of r, nor the pith, which would be found 

 on the opposite side, i.e., beyond m. We have therefore 

 before us a portion of the wood cylinder which reaches to c: 

 c represents the cambium ring, which is made up of thin- 

 walled cells, fitting closely together, without intercellular 

 spaces, and rich in protoplasm. These cells increase in number 

 and become transformed towards the inside, i.e., in the direc- 

 tion of nh, into new wood elements, on the outside into new 

 phloem elements, while the median portion (where the letter c 

 is placed) remains the actual seat of formation of new cells. 



The tissue lying outside the layer of cambium consists 

 chiefly of green cortical cells, in which are imbedded strands 



