TISSUES AND SIMPLE ORGANS. 93 



ing, the cambium at the margin of the injury overflows with callus- 

 parenchyma cells and callus-cortex, gradually covering the injured 

 surface ; finally the callus-tissues meet and the cambial layers again 

 form a closed ring. 



The annual rings in the cortex are not so definitely marked as 

 they are in the wood-tissue. But because of the fact that the second- 

 ary cortex receives an additional layer from the cambium each year, 

 it is not surprising that such annual rings should appear. The 

 annual layer deposited on the outer surface of the cambium is, 

 however, usually very thin, as compared with the layer deposited 

 on the inside. It often happens that there are periodical deposits 

 of thick-walled bast-cells formed. Observation shows, however, 

 that such layers do not agree exactly in number with the annual 

 rings of the wood. . 



By way of comparison of the mechanical and conducting sys- 

 tems in the monocotyledonous and dicotyledonous stems, the follow- 

 ing may be added, though it is in part a repetition : 



Among monocotyledons the mechanical system has its seat 

 either in an independent simple or ribbed bast-ring, with which 

 the conducting bundle (mestome) may be more or less intimately 

 associated, or in a ring or wreath of isolated bast-bundles, or in 

 strong bast-linings which accompany the conducting bundles. 



In the majority of dicotyledonous stems the mechanical system 

 is found within the cambium-ring of the woody tissue ' ' intracam- 

 bial libriform ring." Among dicotyledons, in general, the me- 

 chanical system with its elements ' ' penetrates ' ' the mestome, that 

 is, the conducting system. 



Among dicotyledons there are instructive instances in which a 

 bast-ring or a ring of bast-bundles gives the stem the required firm- 

 ness during its early life-history. Later the mechanical support is 

 transferred to the gradually increasing woody cylinder, while the 

 above-mentioned bast-complexes are gradually displaced by bark- 

 formations (BerberiS) Lonicera, Platamis, Betula, etc.). 



It may be mentioned here that there are monocotyledons (ex- 

 ample, Dracaena) which have a secondary growth in thickness by 

 means of a ring of meristematic tissue. In the case of Draccena 

 we have to deal with fibrous tracheids (similar to those of conifers), 

 which are formed inwardly and united with the leptome in the 

 form of bundles. 



