Metaphyta Ranunculus. \ 79 



cortex, whilst joining these two tissues and running between 

 the fibro- vascular strands lie plates of fundamental tissue to 

 which the name of medullary rays has been given. The 

 tissues of the strand are, moreover, also arranged concen- 

 trically. It is easily possible to differentiate three layers in 

 the strand : internally and next the pith, the xylem, con- 

 sisting of prosenchyma, parenchyma, and vessels. These 

 latter are partly distributed through the wood (dotted ducts) 

 and partly collected at the tips of the wood next the pith 

 (spiral vessels), where the latter form what is known as the 

 medullary sheath. Next we meet with a layer of actively 

 growing tissue, the cambium, composed of parenchyma rich 

 in protoplasm ; and externally the bast, or phloem, which is 

 composed on its inner side of parenchyma, prosenchyma, 

 and sieve-tubes, on its outer aspect of thick-walled prosen- 

 chyma. 



As the stem grows older the strands increase in size, and 

 approach one another so that the medullary rays are di- 

 minished. Moreover, a layer of cambium appears, uniting 

 the cambium regions of the various strands together. The 

 cambium of the strands and the cambium between the 

 strands are known as fascicular and interfascicular cam- 

 bium respectively. The interfascicular cambium has the 

 power of forming xylem on its inner and phloem on its outer 

 side. Thus are formed new or secondary strands, which 

 become wedged in between the primary strands, squeezing 

 and crushing the medullary rays, of which nothing is ulti- 

 mately left save a few thin plates or isolated patches of cells. 

 The cambium of both kinds has, however, another duty to 

 perform, namely, to add new wood and (to a less extent) 

 new bast to the wood and bast already formed. Towards 

 the end of the year when the vitality of the plant is ebbing, 

 the cambium cells divide less rapidly and the cells are much 

 smaller. Again, on the return of spring, when vitality is in 

 full flow, the cells become much larger and divide rapidly. 

 This cessation and renewal of growth form what are popu- 



N 2 



