THE VEGETATIVE ORGANS OF PLANTS. 



281 



the sieve-cells in the overground stem of the potato ; A, 

 J2, cross-section of parts of vascular bundle -4, exterior 

 part towards rind ; J5, interior portion next to pith- 

 cell-tissue inclosing 



sieve- 



which a 



turbid 



the smaller 

 cells, A, B, 

 contain sap 

 with minute gran 

 ules ; , cambium 

 cells \ c, wood-cells 

 (which are absent in 

 the potato tuber ;) d, ^ 

 ducts intermingled 

 with wood-cells. C 

 represents a section 

 lengthwise of the 

 sieve-ducts; and D^ 

 more highly magni- 

 fied,exhibits the fine 

 ly perforated, trans- d 

 verse partitions, 

 through which the 



O 



liquid contents free 

 ly pass. 



Milk Ducts, Be- a 

 sides the ducts al 

 ready described, 

 there is, in many 

 plants, a system of 

 irregularly branched 

 channels containing 

 a milky juice, as in the sweet potato, dandelion, milk 

 weed, etc. These milk-ducts, together with many other 

 details of stem-structure, are imperfectly understood, and 

 require no further notice in this treatise. 



Herbaceous Stems* Annual stems of the exogenous 



55. 



