90 



ORGANOGRAPIIT. 



of Exopjcns, })ut such huiidlcs have no cambnim-laycr on their 

 outside, and have c()nso([ucntly no power of indefinite increase 

 as is the case with exogenous stems. 



Fig. 187. 



Fi(l. 1K7. Transverse section of the stem of the common llccd P'ig. 



IHH. Seialun of the underground stem of n species of Sarsaparillu. a. Epi- 

 rtermul tissue, i.c.t/. The cortical portion, c. Woody zone. /.Medulla 

 or pith. 



There is nothinj^ in tlie internal structure of cndo<;cnous stems 

 hy which we Can ascertain their a;ie as we can those of exo}^enous. 

 It was formerly supposed that the age of a Palm stem was in- 

 dicated hy the anmilar scars produced on its external sin-facc 

 hy the fall of the leaves, each ring hcing su])poscd to he equi- 

 valent to a year's growth (Jh/s. 178 and IST)). 'I'licse rings are 

 however by no means to l)e depended upon as indications of the 

 age ol" sucii trees, for there are frecpiently several rings jn'oduccd 

 in one year, and these again often disappear after having existed 

 for a certain ])eriod. Tlie best means of ascertaining the age 

 of I'alms is by noting their increase in height in one year's 

 growth, and then as such stems grow almost uniformly in suc- 

 cessive years, by knowing their height we can determine their 

 age. This mode however of calcMlating their age is very liable 

 to error, and we cannot thereiore be considcre<l at present to 

 j)Ossess any certain jueans of determining the age of endoge- 

 nous stems. 



C. AcuoQENOus OR AcoTYLEDONous Stem The simi)lcst 



fonn of stem ])resente(l by Aeotyledonous Plants is that of flosses 

 and Liverworts. In their stems we have no true vessels, but the 

 whole is composed of ordinary ])arenchyma, with occasionally a 

 vertical central cord of wood-cells. In the stems of Club- 

 mosses (Li/co/ioilidced), Pcpjierworts (flfdisilntrca), and Horse- 

 tails {/'Ji/itlsrtdced), we have tlie simplest f(»rms (»f acrogcnous 

 st<'ms which contain vessels and the peculiar vascular bumlles 

 (Siiiinltinicous), which are tiieir especial characteristics. The 

 composition of these vascidar bundles and their mode of growth 



