VASCULAR TISSUE. 



51 



leafstalk, or the leaf of an Amaryllis, and gently separating the 

 broken ends ; when the uncoiled threads appear to the naked eye 

 like a fine cobweb. In stems furnished with pith, the spiral ves- 

 sels usually occupy a circle immediately around it. They occur 

 also in the veins of the leaves, and in all parts which are modifi- 

 cations of leaves. More commonly the spire is formed of a single 

 fibre, as in Fig. 45, 46 : it rarely consists of two fibres ; but not 

 uncommonly of a considerable number, forming a band, as in Fig. 

 47. Such Compound Spiral Vessels are to be found in an Aspara- 

 gus shoot ; and are finely seen in the stems of the Banana, from 

 which the fibres may be extracted in large quantities. From the 

 Musa textilis of Manilla, of the same genus as the Banana, these 

 cobwebby fibres are procured and used in the production of the 

 most delicate of textile fabrics. By comparing Fig. 47 with Fig. 

 42, we may readily perceive that the wall of those ducts in Ferns 

 which tear into a band when pulled asunder may have an indis- 

 tinct spiral deposit, composed as it were of a band of fibres that 

 are confluent into a lining, but are individually separated at points, 

 so as to leave interstices in the form of bars, &c. 



61. These ducts or vessels usually have tapering extremities 

 (Fig. 45-47), as in prosenchyma. Like prosenchyma, they vary 

 greatly in length ; some of them are barely oblong or cylindrical, 

 and are manifestly only simple cells, of the same character as the 

 fibrous- walled cells formerly mentioned (46, Fig. 26, 29), which no 

 one would think of calling vessels. Others, 

 though still nothing but single cells, are more 

 prolonged. But those which form tubes of 

 much greater length usually consist (as their 

 development shows), like dotted ducts, of a 

 row of cells formed by multiplication (32-34, 

 and therefore produced from one cell), with the 

 intervening walls obliterated, so as to give a 

 continuous calibre. This origin is well shown 

 in some of the spiral ducts in Fig. 48 (a, , c), 

 which are conspicuously jointed, or composed 

 of a series of cells directly confluent by their 



FIG. 48. A bundle of spiral ducts from the stem of Polygonum orientale, magnified: a, one 

 composed of short cells and with the fibre closely coiled : the next, h, is composed of much 

 longer joints and has a very loose coil : c is short- join ted, and the fibre of the loose coil is oc- 

 casionally forked : d and e show no appearance of joints 'or partitions, and the turns of the 

 spiral fibre are still more remote. 



