. SECONDARY THICKENING. 



This is usually produced by the differentiation and develop- 

 ment of tissue, by means of OPEN FIBRO-VASCULAR BUNDLES. 



I. In Dycotyledonous Stems. 



Illustration: STEM of MOON SEED VINE. (Fig. 27.) 



PREPARATION FIRST : Refer to the permanent preparation 

 made from the stem of this plant representing one year's growth. 

 P. 103. 



PREPARATION SECOND: Stem of the Moon Seed Vine of 

 two or more years' growth. Prepare transections and longitud- 

 inal sections corresponding to those made for Preparation First. 



OBSERVE: 1. The changes in aspect when compared with 

 Preparation First ; (a) The several annular layers of wood 

 formed from the xylem ; (b) each layer in any bundle having one 

 of corresponding thickness in the bundles on the right and left. 



THE ANNULAR LAYERS ; (c) The distribution of spiral and 

 woody vessels ; (d) Of woody fibre ; (e) Tbe slight changes in the 

 Phloem region of the bundles. 



2. THE CAMBIUM RING, of thin-walled cells forming by their 

 division new tissue. (1) Xylem. (2) Phloem. (3) Medullary. 

 (4) New Cambium. 



This ring of cambium is divided into two portions, (1) 

 Fascicular, that included in the fibro-vascular bundle. (2) Inter- 

 fascicular, that between the bundles and included in the JA <//-/- 

 lory Rays. 



4. The large Medullary JRays, composed of cells whose walls 

 have not become thickened, (contrary to the general rule in Medul- 

 lary Bays of several years' growth.) 



