Monocotyledonous Stems 



S3 



is soft and delicate ; but in trees, as Palms, it is much hardened 

 by secondary deposit, forming tvoody parenchyma. 



It follows from this structure that it is impossible for such 

 a stem to increase 

 in thickness after the 

 outer rind has become 

 thoroughly hardened. 



In some Monoco- 

 tyledons of the Lily 

 tribe, as the Aloe and 

 Dragon-tree (fig. 75), 

 there is a provision for 

 the exogenous forma- ^ 

 tion of new bundles, but 

 in a different manner 

 from that which takes 

 place in Dicotyledons. 



A layer of meristem 

 is formed in the outer 

 part of the ground tis- 

 sue, by means of which 

 the stem is increased 

 in thickness, and in 

 which new fibro-vas- 

 cular bundles are 

 formed. 



In Monocotyledons Fig. 75.— Transverse section of stem of Draccena, near 



.1 • i 4.V, apex : ^, epidermis ; k^ cork : r, cortical portion of 



tne sap rises to tne ground tissue; ^, fibro-vascular bundle to leaf: ;«, 



lpnv(^i: t-hmncrll thp ground tissue of centre of stem ; ^-i fibro-vascular 



icdvcd Liiiuugii tiic bundles : jT, menstem zone, developing new fibro-vas- 



Xvlem of the bundles '^'^'^'^ bundles (g') and new ground tissue (st). (After 



and descends through 



the phloem, and apparently partly by the parenchymatous tissue. 



EXTERNAL FORMS OF STEMS. 



A section of the stem usually shows it to be more or less 

 cylindrical in shape ; at other times it is angular, being either 

 triangular, square, five-ribbed, etc. (fig. 76). 



When the stem is herbaceous, and dies down annually, it is 



