36 SCIENCE PRIMERS. [vm. 



of the cambium layer (which lies between the bast 

 and the wood) in spring, in the position indicated; 

 and which after the leaves expand, and are acted upon 

 by light and heat, gives rise to an additional layer of 

 new bast-tissue inside the old bark, and new wood- 

 tissue with vascular tissue amongst it outside the old 

 wood. 



56. Omitting details (such as the formation of layers 

 of cellular tissue outside the bast-tissue), this is the 

 plan upon which the stem and branches of all plants 

 with- two cotyledons are formed. It has been called 

 Exogenous growth, because the bulk of the stem is 

 increased by additions to the outside of the wood. 

 Exogenous plants are hence synonymous with Dicoty- 

 ledons (Pars. 39, 53). 



57. The branch or stem of a dicotyledonous tree 

 or shrub (as the lime) if more than one year old, hence 

 consists, proceeding from the centre, of (a) pith ; 

 (b) layers of wood (with a little vascular tissue), of 

 which the oldest layers are next the pith ; (c) layers 

 of bast-tissue, of which the oldest are next the cir- 

 cumference ; (d) layers of cellular tissue, of which the 

 oldest are next the circumference ; (<?) rays of cellular 

 tissue stretching from the pith to the circumference. 



58. The pith of the centre never grows after the 

 first year ; but the cellular tissue of the outside of the 

 bark often grows by annual additions between it and 

 the bast layers, and of that the barks of the cork, 

 plane and birch trees afford conspicuous examples. 



59. The stem or branch of the butcher's broom, 

 or asparagus (Monocotyledons), has a totally different 

 structure. A cross-cut shows that the whole consists 

 of a cylinder of cellular tissue, traversed by isolated 



