THE VEGETATIVi: ORGANS OF PLANTS. 



281 



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

 _B, cross-section of parts of vascular bundle — A, exterior 

 part towards riad ; JB, interior portion next to pith — a, a, 

 cell-tissue inclosing j\. 



the smaller sieve- 

 cells, A, B, which « 

 contain sap turbid 

 with minute gran- 

 ules; h, 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- 

 verse partitions, 

 through which the 

 liquid contents free- 

 ly pass. 



Milk Ducts.— Be- 

 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 

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