THE LIGNEOUS SYSTEM. 143 



medullary rays. Through these latter the quickening influence of the cambium 

 pervades both wood and bark. 



Vll. Unlimited growth is therefore a characteristic of the exogen- 

 ous stem : for the yearly increments are added to the outside of the 

 wood, and the bark is capable of expansion by lateral growth to any 

 extent. 



712. The peculiar secretions op the plant are generally more 

 abundantly deposited in the bark than in the other parts. Hence the 

 bark is more generally sought for its medicinal and chemical proper- 

 ties. 



713. The endogenous structure. In the cross-section of a mono- 

 cotyledonous stem (corn, palm) there is no visible distinction of bark, 

 wood, pith, or of annual layers of any kind. 



714. It is composed of tissues quite similar to those of the exogen- 

 ous stem, but very differently arranged. The body of the monocoty- 

 ledonous stem consists of parenchyma, within which tissue numerous 

 thread-like bundles of woody matter are imbedded. 



715. These bundles consist each op one or more dotted ducts 

 accompanied by spiral vessels, pleurenchyma, and often cienchyma also, 

 variously arranged in different species. 



716. The toemation op these bundles is dependent upon the leaves from 

 which they may severally be traced downwards, first tending toward the interior 

 of the stem. Further on they recurve outward again, and finally terminate near 

 the surface, there interlacing and combining with their fellows and forming an 

 excessively hard but inseparable rind (false bark). 



TIY. Cleavage difficult. From this entanglement of the fibers the cleavage 

 of endogenous stems is ditBcult or impossible. In jointed stems (culms) this entan- 

 glement occurs only at the nodes (cane, grasses). 



718. The growth of monocotyledon ous stems thus takes place by 

 the addition of the new wood bundles to the interior of the stem, and 

 hence such plants are called Inside-growers or Endogens. 



719. Peculiar forms of the caudex. The rmd of endogenous trees is capable 

 of only a limited expansion. This limit is soonest attained at the base of the stem 

 long before the upper parts cease to enlarge. Consequently such trunks are often 

 seen of equal or greater diameter at the summit than at the base : so the palmetto, 

 com, bamboo. 



720. The acrogenous structure is found in mosses, ferns, and the 

 other higher tribes of the Cryptogamia. The stems advance, beneath 

 or above the ground, full-formed, growing only at the end, hence called 

 Acrogens. 



721. A cross-section of a fern stem shows a body of parenchyma strengthened 

 by an outer zone of fibro-vasoular bundles, the whole invested with a sort of bark. 

 The bundles are precisely similar to those found in the petioles, showing that the 

 stem is the aggregate of the unaltered leaf-bases. (600.) 



