74 ORGANOGRAPHY. 



cells, SO as to form the indefinite vascular bundles which are cha- 

 racteristic of such a stem. Tliese bundles are at first separated, 

 from each other by large intervening spaces of parenchyma 

 Fig. 168. {Jig- 168), but as growth proceeds 



they continue to enlarge, while at 

 the same time new vascular bundles 

 are formed between them, so that 

 they ultimately form at the end of 

 the first year's gro-w-th a zone of 

 vessels and wood-cells round the 

 central mass of parenchyma, inter- 

 rupted only at certain points by 

 cellular projections of this paren- 

 'yk^--::^;'-^^-'.^y^'' chyma, in the form of radiating 



^ r^^^B^^K lines. Tliis zone is also surrounded 



Fig.m. Horizontal section of the V an external layer of cellular 

 first year's shoot of an Exoge- tissue which is Connected with the 

 "rpith: ,'!'ridrmaTU'.'"l: ^^^^ral parenchyma by the radiating 

 Spiral vessels forming tiiemedui- lines already alludcd to. The stem 

 whTch'r the^othe'eiem^iS o? then presents the following parts 

 the vascular bundle. {fig. 168). I. A Central mass of 



parencliyma, m, which is called the Medulla or Pith ; 2. An 

 interrupted zone or ring of wood-cells and vessels, forming the 

 Wood, t; 3. An external zone of parenchyma, or Bark; and 

 4. The radiating lines, r, connecting the pith and the bark, 

 called the Mednllary rays. On the outside of the wood is the 

 cambium ; and the bark is also invested by the epidermis 

 already fully described. Such is the structure of all exoge- 

 nous stems which die annually. 



The stems of plants which live more than one year, as those 

 of trees and shrubs, at first resemble those which are herbaceous 

 or die yearly, except that the wood in such cases is generally 

 firmer and in larger proportion. As growth proceeds in the 

 second year, a ncAv zone of wood is formed on the outside of 

 that of tlie previous year {fig. 169, 2), while at the same time a 

 new fibrous layer is added to the inside of tlie bark, /. These 

 layers arc developed out of the cambium cells c, already alluded to 

 as forming a vitally active layer of cells on the outside of the 

 indefinite vascular bundles which form the Avood of Exogens. 

 The medullary rays (/^. 171. z) at the same time increase by 

 addition to their outside, and thus continue to keep uj) the con- 

 nexion between the pith and the bark. In succeeding years we 

 liave in like maimer new layers of wood and fibrous bark, one of 

 each for every year's growth {fi<i. 169), while the medullary rays 

 also continue to yrow from within outwards. Each succeeding 

 year's growth is therefore essentially a rci>ctition of that of the 

 first year, except as regards the ])ith, so that in old stems wc 

 have no more distinct regions than in those of the first year. 



