FIBRO-VASCULAR TISSUE. 



19 



seems that in this case we find an adaptation peculiarly fitted 

 to accomplish a given end, and it would require not a very great 

 stretch of imagination in conceiving the design of nature in 

 giving to this tube the form she has. It is the only kind fitted 

 to convey nourishment, and give support to the tenderest shoot 

 as it emerges to light. If a common cylindrical tube were 

 used, the great flexures made by such tender parts, under the 

 influence of wind and rain, would be very liable to crush the 

 tube on one side or tear it asunder on the other, as it is well 

 known that a tube cannot be bent without injury, and it is 

 equally well known that a coil may be bent in any direction, 

 and return to its first position uninjured. Here nature, in her 

 wisdom, has adapted organs to the necessity of the case, and 

 she only uses this kind where the above circumstances seem to 

 demand it, as they are never found in any circumstances where 

 they are not terminated with the organ. 



30. The varieties of this tissue, as exhibited by the microscope, 

 are numerous ; but we shall notice only a 

 single kind of its variations. Annular 

 ducts, as they are called, are tubes in which 

 the spires are apparently broken into rings 

 and joined at their extremities. Some- 

 times the rings lie in regular order and 

 in contact with each other, having the ap- 

 pearance of the true spiral vessel, as seen Annular duct 



^ in Fig. 18 a. At another time the rings detached. ni!S " 

 appear separated and irregular, and are Fig. 19. 

 detached from the tube and lying 

 withrings lengthwise in it, as seen in Fig. 18 6. 

 broken! d and These appearances may be seen in the 

 stem of the Impatiens, and other forms 

 will be readily detected in the same plant. 



31. Rayed Vessels. — These vessels are formed 

 by the rings being broken into regular pieces 

 and arranged in regular order, like the rounds 

 of a ladder, and on that account called scalari- 

 form by some. The tubes are generally hexagonal prisms, 

 Fig. 19. They are found in ferns and roots of plants. 



Fig. 18. 

 b 



r4 



<a>°c 



Fig. 18. 

 a 



Scalariform vessels. 



80. What are annular ducts?— 31. "What are rayed vessels 



