MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS. 



365 



but their structure may be more easily displayed by cutting round, but 

 not through, the leaf-stalk of the Strawberry, Geranium, etc., and then 

 drawing the parts asunder. The membrane composing the tubes of the- 

 vessels will thus be broken across; but the fibres within, being elastic, 

 will be drawn out and unrolled. Spiral vessels are sometimes found to 

 convey liquid, whilst in other cases they contain air only; the conditions 

 of this difference are not yet certainly known. 



3G3. Although fluid generally finds its way with tolerable facility 

 through the various forms of cellular tissue, especially in the direction of 

 the greatest length of their cells, a more direct means of connection 

 between distant parts is required for its active transmission. This is 

 afforded by ducts, which consist merely of cells laid end to end, the par- 

 titions between them being more or less obliterated. The origin of these 

 Ducts in cells is occasionally very evident, both in the contraction of 



FIG. 249. 



"Fas. 248. 



Section of Coniferous Wood 

 in the direction of the Fibres, 

 show ing 1 their 'glandular' dots : 

 - a , a, a. Medullary Rays cross- 

 ing the fibres. 



Longitudinal section of stem of Italian Reed :a t 

 Cells of the Pith; 6, Fibre-vascular bundle, contain- 

 ing 1, Annular duct; 2, Spiral duct; 3, Pitted duct, 

 with Woody fibre; c, Cells of the integument. 



their calibre at regular intervals, and in the persistence of remains of 

 their partitions (Fig. 263, I, 1} ; but in most cases it can only be ascer- 

 tained by studying the history of their development, neither of these 

 indications being traceable. The component cells appear to have been 

 sometimes simply membranous, but more commonly to have been of the 

 fibrous type ( 357). Some of the ducts formed from the latter (Fig. 

 249, 2) are so like continuous spiral vessels as to be scarcely distinguish- 

 able from them, save in the want of elasticity in their spiral fibre, which 

 causes it to break when the attempt is made to draw it out. This rup- 

 ture would seem to have taken place, in some instances, from the natural 

 elongation of the cells by growth; the fibre being broken up into rings, 

 which lie sometimes close together, but more commonly at considerable 

 intervals; such a duct is said to be annular (Fig. 249, 1). Intermediate 



