22 A USTRALIAN BOTANY. 



tissue consists of a number of very small cells massed 

 together, and containing a colouring matter called chloro- 

 phyll, generally green, which will be further mentioned in 

 the lesson on ' Leaves.' Through these cells the matters 

 absorbed by the root pass to nourish the plant. As the 

 exogen becomes older, successive layers of woody tissue 

 are yearly formed in rings round the pith or heart of the 

 stem. Each successive ring is formed outside the previous 

 one, those nearest the heart of the stem being the earliest 

 formed. Hence the term exogen, signifying ' outside 

 grower.' Examine a cross section of the stem of a gum- 

 tree or wattle as an example. See Plate II. fig. i, p. 20. 



EXDOGEXS OR MOXOCOTYLEDOXS. 



(Producing one seed-leaf in early growth.) 

 Though not so lars;e in number as the exo°;ens, this 

 class includes many beautiful and valuable plants. The 

 graceful tribe of palms, the screw-pines, club-lilies, and 

 other genera found in Australia are monocotyledons. The 

 stems of endogens increase in diameter by vascular bundles 

 — fibrous vessels growing inwards, towards the stem. (See 

 Plate II. fig. 2, p. 20.) Hence the term endogen or 

 ' inward-grower.' Endogens have no annular rings, heart- 

 wood (duramen), or true bark. Generally speaking, their 

 leaves are continuous with the stem. The softest part of 

 the stem is the centre, which is sometimes hollow ; the 

 hardest part is the outside or rind. In some endogens, 

 such as the Dracaena or Cordyline, the outer skin never 

 hardens, consequently the stem sometimes becomes 

 bulky. 



Bark is the outer covering of exogens. It varies greatly 



