128 GROWTH. 



197. In those tropical countries in which the wet and 

 dry seasons are well marked from each other, so will be 

 the periods of vegetable vigour, and the separation be- 

 tween the zones of wood be easily seen, but in others 

 in which these seasons are not so well marked, neither 

 will be the periods before mentioned, and one zone of 

 wood will be found running into the other so that no 

 line of demarcation can be seen. It has likewise been 

 said that in certain tropical situations where the atmos- 

 phere and earth are constantly moist during the whole 

 year, two periods of vegetation are known, and a har- 

 vest takes place in summer and in winter, and that there 

 two zones of wood will be annually deposited in plants. 

 In the Island of Lucon Meyen saw, as he supposed, evi- 

 dences of such a supposition. Decaisne has said that 

 each zone of wood in Menispermaceae is the growth of se- 

 veral years. Again, the zones are not always concentric 

 to the pith ; in plants of southern countries the eccentri- 

 city is greatest, especially in those of damp situations. 

 In some cases the amount of eccentricity is so great that 

 the divergent portion of the zone ceases to have the 

 pith within its circumference. 



198. The origin of the Medullary rays which tra- 

 verse the woody layer, must be looked for at the first 

 period of the growth of the stem ; they are formed out 

 of the parenchymatous cellular tissue, which is placed 

 between the pith and bark of the young plant, and which 

 from the separation of these two from each other by the 

 deposit of woody matter between them is extended, and 

 flattened into radiating plates. 



199. Kieser and Meyen make two sort of medullary 

 rays ; the large ones or those running from the pith to 

 the bark ; and small ones found in many stems, and not 

 keeping up a connection through the centre to circum- 

 ference, but limited in their extent. 



200 Besides a growth connected with the woody 

 layers of plants, there is likewise one of the bark to be 

 observed, but here the new matter instead of being de- 



