18 WOODY TISSUE. 



seriously if at all impeded by the partitions which lie across the tube, as would at first 

 r h U P pear for even should such partitions be perfect, they readily penmt the proper 

 S to niter through them. The great size of this kind of tissue, and the large quantity 

 of fluid which it contains, render it imperative that it should be supported ^ ct ^ e3 

 more resisting than its own. For this reason it is always found surrounded by bundles 

 of stem- woody tissue. Another function assigned to it in later life is that of conveying 

 air into the interior of the plant. This occurs when the walls of the cell or tube have 

 become imperfect, and would permit contained fluid to pass out of them ; and then th 

 fluid disappears, and its place is supplied by air. A third, and not less important duty, 

 is that of a depository of the secretions of the plant. This only occurs when the tree 

 is mature, and the central parts of the trunk, which are not then devoted to the rapid 

 conveyance of fluid for the purposes of perspiration. The deep-colouring matter of 

 rose-wood and mahogany, and all similar trees, is chiefly found in this tissue. 



From the above remarks it will be evident that bothrenchym is a tissue of great 

 interest and importance, and is seen in its integrity only in the early life of a plant. 

 Its large size, thin walls, and active functions, seem to predispose it to injury ; ai 

 therefore such tubes have the duty assigned to them of conveying air, or of storing 

 up secretions which do not circulate. 



Gridiron Tissue.-Under the term of gridiron tissue, Professor Quekett has 

 described an interesting structure, oftentimes met with at the end of 

 the ducts of pitted tissue. It consists of a series of bars which pass 

 transversely across the tube, and occupy the position of the usual 

 transverse septum. It is probably not a distinct structure, but only 

 the remains of the original septum. We have met with fine examples 

 of it in several trees, but more particularly in the alder and white 

 birch (Betula alba). A similar condition has also been observed in 

 a fossil palm found at St. Vincent's. 



Pleureiichym, or Woody Tissue. The tissue most closely 

 allied to bothrenchym, and yet widely removed from both it and 

 cellular tissue, is pleurenchym, or woody tissue. This constitutes 

 the mass of the stems of our forest trees, and is thus of the utmost 

 social use to man. It is, also, found in all young and tender 

 shoots, and in bundles in the stems of all, even the most delicate O f a dotted duct in 

 flowering plants. Its peculiar characteristic is that of great tena- the alder ( Alnus )- 

 city and power of resistance, and for this its structure is admirably adapted. As these 

 characters are opposed to those of bothrenchym, we are prepared to find a tissue dif- 

 fering widely from that large and wide structure. The contrary is found in woody 

 tissue, for it consists of bundles of very narrow fibres, with tapering extremities, and 

 so placed end to end that the pointed ends overlap each other. Each fibre is very 

 short, and the partitions which result from the apposition of the fibres, end to end, 

 do not interfere with the circulation through them. Moreover, the tube is not com- 

 posed of simple thin membrane only ; but, in addition, has a deposit within it, which, 

 without filling the tube, adds very greatly to the strength of the fibre. Perhaps we 

 have here as good an illustration of the wisdom and power of the Creator as can 

 readily be produced nz., an arrangement whereby the greatest strength and power 

 of resistance and elasticity shall be obtained, and at the same time the functions of cir- 

 culation uninterruptedly maintained. The strength is mainly due to the shortness of 

 each fibre, the connexion by apposite ends of many fibres almost in one direct line, 



