Organization of Leaves,. 7 



appear transparent in their axes, and they are seen to ana- 

 stomose with each other wherever they meet, and not to cross 

 or to form knots in any manner. As these characters are 

 suited to vessels filled with a transparent fluid much rather 

 tlian to simple solid fibres, I have always considered them 

 as vessels, and shall give them that name in future. The 

 fineness and transparency of these vessels induce me to 

 consider them as lymphatic vessels." 



Hedwig observed the same reticulation, which he says is 

 formed by vessels to which he has given the name of ciiti^ 

 culw hpnphatic vessels. 



These two authors have committed an error in taking for 

 vessels the lateral faces of the utriculae which form the sur- 

 face of the leaves ; but this error, which is owing to an op- 

 tical illusion, was easily committed. If the pellicle of a 

 leaf, indeed, be removed, and be examined with the micro- 

 scope, it will readily be conceived, that if the utriculae 

 which compose this pellicle be viewed from above, their 

 lateral or perpendicular faces will appear shortened ; and 

 besides, as these membranes may be more or less inclined 

 on account of their flexibility, the result will be, that the 

 lower ridges will appear to be close to the upper correspond- 

 ing ones, which will give the illusion of vessels of different 

 diameters, the transparency of the axes of which will depend 

 on that of the membrane comprehended between these ridges. 



Fig. 1 will serve to give a better idea of the optical illu- 

 sion which I have here endeavoured to explain. 



As the lateral faces of the exterior utriculae thus present 

 themselves under the appearance of vessels, it is seen that 

 the result will be a kind of reticulation, the meshes of which 

 will vary according to the form of the utriculae : for ex- 

 smiple, they will be rectangular, hexangular, or irregular, 

 according as the utriculae represent a parallelopipcdon, a 

 hexaedral prism, or any other irreg-ular form. 



From what has been said, I entertain no doubt that if 

 the authors I have ouoted had examined the pellicle of the 

 leaves in profile or in vertical sections, instead of observing 

 them in front or from above, they would have found that 

 what they take for vessels are merely the lateral faces of 

 the utriculae. 



C. Mirbel in his memoir above mentioned seems to 

 ascribe the errors committed on this subject to the same 

 cause. He says, " The difiiculty of observing, by the help 

 of the microscope, the relation which exists between the 

 plane* placed in different directions, and at unequal di- 

 A 4 stances, 



