THE VASCULAR TISSUE. 



27 



Ducts are tubes with conical or rounded extremities, and their sides marked by 

 transverse lines or bars. Their size is about twice that of spiral vessels. 

 Their appearance is very various, and depends upon the direction of the 

 spiral fibre which assimilates ducts to spiral vessels, or the presence of 

 other internal deposits, which renders them not unlike pitted tissue. 



When the spire is so arranged as to differ from that of the spiral 

 vessel only in that it cannot unroll, the vessel is termed a closed duct. 

 When it is broken up at intervals, so that single coils shall be" detached, 

 the term annular is applied (Fig. 67), and properly represents the rings 

 which are so commonly found in ducts. This form is said to be due to 

 the rapidity of the growth, whereby the fibre is carried along more 

 rapidly than the membrane can be produced. 



The reticulated duct is perhaps the most interesting of the various 

 kinds of ducts, and appears to be formed either by two fibres wound in 

 opposite directions so as to cross each other, or by a single fibre which 

 breaks and anastomoses at intervals. The characteristic feature is that of 

 a net-work. All these various forms of duct, and also other modifica- 

 tions, may be found in the stem of a full grown 

 garden balsam. The succulent stems of herbaceous 

 plants are the more common positions in which 

 ' ducts are found ; but they are abundantly met 



with in the softer kinds of wood, as of the lime- 

 tree (Tilta), willow (Saliz), or birch (Betula). 



We cannot omit to refer again to the analogies which exist 

 in the structure of animals and vegetables. Thus, in the animal Fig. 68. The tracheae 

 kingdom, we have a tube which very closely resembles a spiral of & th w C 'er beetle 1 

 vessel, viz., the tracheas of insects. This is clearly shown in having many of the 

 the accompanying figure of the Dyticus (Fig. 68), which repre- 

 sents a tube made simply of a fibre inclosed by membrane. 



It is unnecessary to refer to all those forms of duct in which we find a secondary 

 deposit so arranged as to give the appearance of pits^ since we have already considered 

 similar structures under the head of Bothrenchym (pp. 7 and 17). 

 But there is one not described as yet viz., the Scalariform or ladder 

 duct. This is so called from the resemblance which the transverse 

 lines bear to the rounds of a ladder. The scalariform duct is of con- 

 siderable size, and usually six-sided, and has a deposit so arranged, 

 on its inner side, that either its presence or its absence causes 

 certain transparent lines to appear at very regular intervals. In 

 some instances so many as twelve sides have been observed ; but 

 whatever may be the number of sides, they are separated by 

 clearly defined perpendicular lines. The transverse bars do not 

 Fig. 69. Scalariform P ass 9*>ite so far as the boundary line of the side a circumstance 



vessel, showing the -which gives a greater degree of resemblance to the figure of a 



transverse oars on . . . 



nine sides, and the ladder. As there are transverse translucent spaces of about equal 



open character of the size and at equal distances, there will, of course, be alternate 



transverse and equal bars separating these spaces. These bars 



are continued with the] boundary line of the side ; and, upon the whole, it appears 



probable that the deposit has been placed at these points, and that the translucent 



