VASCULAR TISSUE. 35<) 



enlarged by the deposition of new matter around it. This 

 becomes a point originating another fibre or branch, which 

 becomes developed by the successive attraction of granules 

 into bead-like strings, taking a contrary direction to the 

 original fibre, forming a cross-bar, or ramifying, thereby 

 causing the appearance by which the vessel is recognised. 



Jn the exogenic vessel, the development of fibre proceeds 

 in the same manner as in the last example ; but the vessels 

 will be seen to be dotted with a central mark, usually of a 

 red colour, which, when viewed under high power, may be 

 thought to resemble a minute garnet set in the centre of 

 each dot. This red colour is owing to the dot being 

 somewhat hollowed or cupped, and the centre only thin 

 membrane. These vessels are 'best seen in the young 

 shoots of the Willow. In the endogenic vessel the con- 

 necting branches are given off beneath each other, so that 

 the dots, which are rounded, are arranged in longitudinal 

 rows ; but in the acrogenic, or scalariform, in which the 

 vessels are generally angular, and present distinct facets, 

 the branches come off in the same line, corresponding 

 generally to the angles of the vessel ; the spaces left 

 between are linear instead of round. 



Mr. E. Quekett affirms, in opposition to the views enter- 

 tained by Mirbel, Richard, and Bischoff, "that the dots 

 left in these several vessels are not holes, neither do they 

 consist of broken-up fibre, but are the membranous tubes, 

 unsupported by internal deposit ; and on account of the 

 extreme tenuity of the tissue, and the minute space between 

 the fibres, the light in its transmission becomes decomposed, 

 and appears of a greenish-red hue. The structure of the 

 dot is best seen by examining the broken edge of any such 

 vessels, when it will be found that the fracture has been 

 caused by the vessel giving way from one dot to another,, 

 so that the torn edge of the membrane can be observed in 

 each dot." 



PREPARATION OF VEGETABLE TISSUES. 



The proper mode of preparing and preserving vegetable 

 tissues is a matter of some importance to the microscopist; 

 we therefore propose to add a few general directions for 

 the student's guidance. 



