374 



STRUCTURE OF PHANEROGAMIC PLANTS. 



FIG. 



played, by cutting round, but not through, the leaf-stalk of the 

 Strawberry, Geranium, &c., 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. 



233. 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 its cells, a more direct 

 means of connection between distant parts is required for an 

 active circulation. This is afforded by what has been termed 

 Vasiform tissue, which consists merely of cells laid end to end, 

 the partitions between them being more or less obliterated, so 

 that a continuous Duct is formed. The origin of these ducts in 

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

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

 of their partitions (Fig. 175, b b) ; but in most cases it can only 

 be ascertained 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 possessed the fibrous type ( 228). 



Some of the ducts formed 

 from the latter (Fig. 162, 2) 

 are so like continuous spiral 

 vessels, as to be scarcely 

 distinguishable from them, 

 save in the want of elasticity 

 in the spiral fibre, which 

 causes it to break when the 

 attempt is made to draw it 

 out. This would seem to 

 have taken place, in some 

 instances, from the natural 

 elongation of the cells by 



frowth; the fibre being bro- 

 en up into rings, which 

 sometimes lie close together, 

 but more commonly at con- 

 siderable intervals ; such a 

 duct is said to be annular 

 (Fig. 162, i). Intermediate 

 forms between the spiral and 

 annular ducts, which show 

 the derivation of the latter 

 from the former, are very fre- 

 quently to be met with. The 

 spires are sometimes broken 



up still more completely, and the fragments of the fibre extend 

 in various directions, so as to meet^and form an irregular net- 



Longitudinal section of stem of Italian Seed : a, 

 cells of the pith; b, fibro-vascular bundle, containing, 

 1, annular duct; 2, spiral duct; 3, dotted duct, with 

 woody fibre ; c, cells of the integument. 



