FTBRO-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 ti"ue spiral vessel, as seen 



Fig. 18. 

 h 



^d 



'S>'^ 



^ 



Annnliir duct 



in Fit?. 18 a. At another time the rinfjs detucbej.*' ""^' 



& — — -- — — ^ 



appear separated and irregular, and are 



^1 detached from the tube and lying 



With rings length wisc in it, as seen in Fig. 18 6. 



broken!'^ ''"'^ Thcsc appcarauccs 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 arran<2:ed in reg-ular order, like the rounds ^ , ., 



„,--* ,0- ' ,,, ,. Scalunform vessels. 



01 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. 



80. What are annular duots ?— 31. What are rayed vessels 



