VEGETABLE TISSUES '. — DUCTS. 403 



occurring in the stem of Endogens : thence they' proceed in each 

 case to the Leaf-stalks, through which they are distributed to the 

 leaves. By careful dissection under the Microscope they may be 

 separated entire ; but their structure may be more easily displayed 

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

 Geranium, &c, and then drawing the parts asunder. The Mem- 

 brane 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. 



296. Although fluid generally finds its way with tolerable facility 

 through the various forms of Cellular Tissue, especially in the direc- 

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

 connection between distant parts is required for its active transmis- 

 sion. 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 remains of their partitions (Plate xiii. , 

 fig. 2, 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 (§ 291). Some of the Ducts formed from the latter (Fig. 

 211, 2) are so like continuous Spiral Vessels as to be scarcely dis- 

 tinguishable 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 rupture 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. 211, i). Intermediate forms 

 between the Spiral and Annular Ducts, which show the derivation 

 of the latter from the former, are very frequently 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 network lining the Duct, which is then said 

 to be Reticulated. The continuance of the deposit, however, gra- 

 dually contracts the meshes, and leaves the walls of the Duct 

 marked only by pores like those of Porous Cells (§ 290); and canals 

 upon this Plan, commonly designated as Dotted Ducts, are among 

 the most common forms of Vasiform tissue, especially in parts of 

 most solid structure and least rapid growth (Fig. 211, 3). The 

 Scalariform Ducts of Ferns (§ 278) are for the most part of the 

 spiral type ; but Spiral Ducts are frequently to be met with also 

 in the rapidly-growing Leaf -stalks of Flowering-plants, such as the 



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