254 Pkyfotomy in the Eighteenth Century. [BOOK n. 



(1784); but he treats these topics at greater length in his 

 treatise ' De fibrae vegetabilis et animalis ortu,' published 

 in 1789, and known to the author of this work only imper- 

 fectly from quotations in later writers. Hedwig's figures of 

 histological objects appear to be better than those of any 

 of his predecessors ; they show that he used strong magnifying 

 powers, and that his glass had a clear field of sight. His 

 defect lay in preconceived opinions and hasty interpretation 

 of what he observed. In order to refute Gleichen's view 

 of the stomata in ferns, he demonstrated the existence of these 

 organs in many phanerogams, and observed the opening of the 

 slits, which he named ' spiracula.' On the epidermis which he 

 had removed for the purpose of these observations he saw 

 plainly the double contour lines bounding the epidermis-cells, 

 and therefore the cell-walls, which are at right angles to the 

 surface. These he took for a particular form of vessel, and 

 called them * vasa reducentia ' or ' lymphatica,' and afterwards 

 'vasa exhalantia,' and he thought that he had found them again 

 in the interior of parenchymatous tissue, evidently taking the 

 places where three wall-surfaces meet for vessels such vessels 

 he also saw in the milk-cells of Asclepias, described in 

 1779 by tne elder Moldenhawer, who seems himself to have 

 regarded even the intercellular spaces in the pith of the rose as 

 equivalent to these milk-cells. The word vessel even in the 1 8th 

 century was used in such an indefinite manner, that the broad 

 air-tubes of the wood and the finest fibres were called vessels. 

 Hedwig's idea of the construction of spiral vessels was strange 

 enough ; he took the spiral band itself for the vessel, and 

 supposed it to be hollow because it is coloured by absorption 

 of coloured fluids; in those spiral vessels in which the turns of 

 the spiral band are distant he saw, it is true, the delicate original 

 membrane which lies between the turns, but he supposed that 

 it lay inside the spiral band, which was wound round it on the 

 outside. On the second plate of the first part of the ' Historia 

 Muscorum ' he even figures the network of ridges which the 



