SECT. I. ORGANOGRAPHY AND GLOSSOLOGY. S3 



which is gradually organised, and ultimately separates 

 into two layers one making an addition to the wood, and 

 the other to the bark, which had been previously formed. 

 Hence a layer of new wood forms a ring round the old 

 wood, and a layer of new bark round the new wood ; 

 whilst the old layer of bark, being necessarily thrust out- 

 wards, is ruptured and withers, though it still continues 

 to form an outer coat over the whole stem. A layer of 

 fresh wood and another of fresh bark are in this way de- 

 posited every year ; and in many cases, we may ascertain 

 the exact age of a tree by the number of the concentric 

 zones observable upon making a transverse section of its 

 stem. Thus, in fig. 24., a is the pith, b represents three 

 layers of wood, and c an 2* r.'^^-TrT-"^-^ 



equal number of layers ^ 



of bark. Besides these /|g| 

 concentric zoned appear- 

 ances on the surface of * 

 the section, there are also 

 other traces running in fc 

 straight lines, radiating | 

 from the centre to the 

 circumference, which are formed of cellular tissue, 

 and termed " medullary rays." Either of these three cir- 

 cumstances, then the existence of a pith, the appearance 

 of concentric zones, or the presence of medullary rays 

 affords a sufficient characteristic by which we recognise 

 the structure of dicotyledonous plants. The plants of 

 this class are further named "Exogense," from the cir- 

 cumstance of their stems increasing in thickness by fresh 

 materials, which are arranged "externally" with respect 

 to the old layers. The oldest and hardest parts of such 

 stems lie towards the centre, as may be readily seen in 

 any tree growing in our temperate zone. 



(35.) Monocotyledones, or Endogence. 



(1.) Structure of the Seed. 



The general structure of the seeds of this class may 

 be exemplified by an examination of a grain of Indian 



