1 74 FIBRO- VASCULAR B UNDLES. PART n. 



circulation of the sap is rapid, and the perspiration 

 great. The sap in this crude state passes freely through 

 the partitions, being probably a dialy sable liquid; but in 

 the autumn, when the sap ceases to rise, the septa are 

 either absorbed or destroyed, as appears from the 

 fragments of them that sometimes remain, and then 

 the ducfcs become filled with air, which they convey to 

 mature the sap in the leaves and all parts of the plant. 

 Some of the vascular ducts have very narrow parallel 

 fibres of a bluish colour twisted in a more or less elastic 

 spiral from end to end of their internal surface, which 

 in by far the greater number of cases turns in the same 

 direction as a left-handed screw. In some ducts they 

 merely cross the inner wall of the cell at regular 

 distances as circles. The reticulated form from the 

 crossing of right and left-handed spirals is still more 

 frequent than the simple spiral ; there is scarcely a plant, 

 from the mosses upwards, in which that structure cannot 

 be found. 



In a vast majority of cases the secondary internal 

 membrane of some of the ducts is perforated by orifices 

 of numerous forms, sometimes irregularly, and some- 

 times in a regular pattern like a sieve, and on that 

 account they are called the pitted tissue. 



In the stem of a tree the vascular cylinders alternate 

 with cylinders of woody fibre, as may be seen in a section 

 perpendicular to the axis, in which the two tissues form 

 a series of alternate rings. The woody or ligneous 

 tissue, which gives strength and solidity to all vegetable 

 structures, consists of bundles of nearly parallel spindle- 

 shaped tubular fibres, having their attenuated extremities 

 applied end to end to the extremities of those above and 

 below them, so that they form groups of nearly straight 

 lines ; but although the ends of these tubular fibres 

 overlap each other they do not prevent a free circulation 

 of the sap. The different layers of these combined 



