136 MINUTE STRUCTURE OF THE STEM. 



tion as have been derived from the leaves at that point ; and 

 since these bundles do not possess a cambium laj-er, they have 

 no power of increasing in size. The only changes therefore to 

 be looked for in the stem of a palm from year to year are those 

 in the ragged extisrior from which the leaves fall, and the pos- 

 sible increase in firmness of the individual elements of the older 

 bundles. The stems of most palms are as thick when they begin 

 to ascend from the ground as thej' will afterwards be, their bun- 

 dles earl}- becoming permanent tissue throughout. 



388. The presence of obscure nodes in the stem may com- 

 plicate its structure somewhat by the introduction of horizontal 

 interlacing bundles ; but there is in these cases, as in the former, 

 no provision for increase in thickness. 



389. In some monocotj-ledonous stems new bundles can arise 

 in a merismatic layer just within the cortex, and therefore cause 

 an increase in the diameter of the stem. 



A similar mode of increase in thickness is met with in the 

 stems of many dicotyledons ; as those of Nyctaginacese, many 

 Chenopodiacese and Amarantacese, etc. Secondary bundles are 

 formed in a merismatic layer outside the primary bundles, and 

 in contact with their liber. 



390. The secondary structure of normal dicotyledonous stems 

 (see 369) is easily understood when it is remembered that the 

 cambium of their priniar}' bundles possesses the power of form- 

 ing the following kinds of tissue : a, new wood on the outside 

 of that which was last produced ; 5, a layer of new liber ; c, fresh 

 cambium for subsequent activity ; and cf, continuations of the 

 medullar}' rays. 



The cambium layer in the stems of most dicotyledons is com- 

 posed of extremely delicate, thin-walled cells, which are filled 

 with protoplasm and building materials. In th^ spring, when 

 the bark is readily stripped from the wood, this laj'er appears as 

 a thin film of mucilaginou^ matter, showing, to the naked eye, no 

 cellular structure. In the case of such plants as the maple, 

 birch, and pine, this juicy mass possesses a very sweet taste, 

 owing to the large amount of organizable nutrient matter which 

 it contains. 



391. The cambium layer exposed by removal of the bark soon 

 dies, and of course all further increase in diameter is impossible 

 unless the wound is healed in some way (see 421). 



392. The growth in size of the stems of normal dicotyledons 

 depends therefore upon the existence and activity of cambium 

 cells between the wood and bark. The juxtaposition of the 



