364 MANUAL OP BOTANY 



■varying quantities of cortical tissue, perieyole, and bast. It is 

 consequently extremely complex. The phellogeus often form 

 not only ordinary cork cells, but mixed with them cells thickened 

 sclerenchymatously. 



In some cases the bark is the product of a single phellogen 

 which continues its activity for many years. A very thick ex- 

 ternal layer of cork is thus formed, as in the cork oak. In 

 other cases a new phellogen is formed every few years, deeper in 

 the tissue than the previous one. 



These formations of phellogen may be cylindrical, cutting off 

 regular zones or cylinders of tissue. Frequently, however, they 

 dip into the other tissues somewhat irregularly, and often in- 

 tersect each other, giving rise to the separation from the stem of 

 irregular sheets of bark (fig. 746). These sometimes, as in the 

 Plane tree, are shed from the trunk in large patches. ^V' e dis- 

 tinguish the latter case from the former by the terms scrile 

 bark and ringed bark respectively. Ringed bark is generally 

 ruptured longitudinally somewhat irregularly, and presents the 

 appearance of grooves or furrows, often of considerable depth. 



Besides this formation of bark, serving as a normal protection 

 to the internal tissues, another tissue is developed in cases where 

 an injury occurs to woody plants. This is known as callus, and 

 consists of ordinary parenchyma, which ultimately becomes 

 covered with cork. It is only peculiar in its mode of origin, the 

 cells which are adjacent to the injured ones becoming merismatic. 

 There is not much difference between this formation and that of 

 the covering of exposed surfaces by cork as already described 

 (page 340). If in a stem or root the wound extends -to the 

 cambium, the callus originates at its sides as before, but cork and 

 new cambium are formed in it, the latter joining the cambium 

 which was injured, and growing over and closing the wound. If 

 any object becomes impacted in the wood by the injury, the new 

 formation grows over it and completely encloses it. 



Turning from transverse sections to study the structure as 

 revealed by longitudinal ones, this type of stem shows at the 

 summit a small-celled meristem, as already described, forming a 

 conical apex on which small lateral protrusions arise in acropetal 

 succession. The division into nodes and internodes is visible at 

 a very early period, the small protrusions or cushions arising at 

 the former. Each protrusion is the rudiment of a leaf. It con- 

 sists of a small outgrowth of the periblem covered by the young 

 epidermis. The plerome or central portion of the stem does not 

 contribute to its formation. 



