ORGA^^S OF NUTRITION. 



growers). In a Palm stem no such distinction of parts can be 

 noticed {fiy. 164), but upon making a transverse section we 

 Fig. 163. 



7 i Fiq. 164. 



Fig. 163. Transverse section of an Oak-branch six years old. m. The me- 

 dulla or pith, ce. The bark ?-. The wood. 6. Medullary rav* Fig. 



Ifi4. Transverse section of the stem of a Palm. m. The cellular substance. 

 /. The fibro-vascular bundles. The whole being invested by a rind or 

 false bark, h. 



observe a mass of cellular substance, m, distributed throughout 

 it, and the fibro-vascular arranged vertically in this in tlie 

 form of separate bundles, /, which have no tendency to form 

 layers of Avood. This structure is called Endogenous (from two 

 Greek words signifying inside growers), as such stems grow 

 by the addition of neAv matter on their inside. Exogenous and 

 Endogenous stems are those therefore which are characteristic of 

 Flowering Plants, If we now r- i c^ ► 



turn our attention to Flowerless 

 Plants, and make a transverse 

 section of a Tree-fern {fig. 1 65), 

 we observe the centre to be 

 either hollow or filled Avith 

 parenchymatous cells, m, the 

 fibro-vascular system being ar- 

 range d in irregular sinuous 

 plates around this, v v, form- 

 ing a continuous or interrupted 

 circle near the circumference, 

 which consists of a rind, e, 

 inseparable from the wood 

 beneath. This structure is 

 termed Acrogenous (from two 



Greek W^ords signifying sum- Fig 165. Transverse section of the stem of 

 xi^"'^i a Tree-fern. m. Parencliymatous cells. 



mit growers), because such a -R.l.ich are wanting in the centre, v, v. 

 Stem grows only by additions libro-vascular bundles e. Eiud. 



to its summit. 



According to the views first propounded by Schleiden, the 



F4 



