894 A TEXTBOOK OF THEORETICAL BOTANY 



old permanent tissues, which occurs in regeneration growth after wounding. 

 Such new developments show a definite directional guidance from point to 

 point, often around obstacles, which strongly suggests the action of a diffusing 

 hormone. This is also probably true of new, adventitious, vascular differentia- 

 tion, as in the case of Cyclamen (see p. 891). 



The cambium, like other meristematic tissues, does not itself undergo 

 any marked change throughout its existence. The only visible indication of 

 age is a gradual increase of the average length of the cambial cells and of the 

 tracheids formed from them, an increase which may continue for a number 

 of years from the beginning of cambial growth. Eventually, however, a steady 

 state is reached and no further change in length occurs. 



In the most pronouncedly herbaceous plants there may be no interfascicular 

 cambium, and the separate bundles themselves have little or no cambial 

 growth, while in the most extreme herbaceous types there is not even a 

 continuous ring of procambium. Indeed the annual, monocarpic habit of 

 growth, limited to a single growing season, is probably brought about by the 

 absence or early disappearance of cambium and the consequent lack of the 

 power of extended growth. 



Monocotyledons arc generally supposed to have no cambium in their 

 bundles, but the metaxylem elements are often in radial rows, which is 

 characteristic of cambial growth, and they probably resemble the annual 

 Dicotyledons in having cambia that are very short lived. The specialized 

 cambia of some Monocotyledons which undergo secondary thickening are 

 dealt with on page 912. ■ 



In connection with the cambium we may briefly mention .the abnormal " 

 tissue formation known as callus, ^^hich is associated with wounds and 

 similar injuries. The name is applied to irregular tumour-like outgrowths, 

 due to a resumption of meristematic activity called out by the injury. Any 

 living tissue may contribute to this development, but where the cambium is 

 involved, its contribution is much the greatest. Thus a callus ring arises on 

 the cut ends of branches or old roots, which is almost entirely due to growth 

 from the exposed end of the cambium and which spreads until the wounded 

 surface is completely covered. This new tissue is almost entirely 

 parenchymatous, and there is no regular differentiation of tissues, as in 

 normal cambial growth. At the most a few irregular scattered tracheids 

 or sclereids may be formed, embedded in the mass, while the outermost 

 cells may become corky. 



8. Secondary Vascular Tissues : Phloem. The metaphloem, including the 

 secondary phloem, if any, consists of sieve tubes, their companion cells and 

 phloem parenchyma. The companion cells, originally sister cells of the 

 sieve tube cells, usually divide transversely more than once, so that they are 

 individually much shorter than the sieve tube segments. The phloem 

 parenchyma cells are variously shaped, and may be so long and narrow that 

 they scarcely justify their name. 



Mingled with these elements in many plants are phloem fibres, either 

 scattered or, as in Tilia (Fig. 879), in tangential bands alternating with the 



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