CHAP, iv.] of the Cell-tissue in Plants after 1845. 343 



the perforation of the walls, believing that the appearances 

 were due to lattice-like thickenings of the cell-walls; he 

 proposed therefore to call Hartig's sieve-tubes latticed cells. 

 Then Nageli showed in 1861 that in some cases at least 

 there can be no doubt that the walls are actually perforated, 

 and that the sieve-plates serve for the passage of mucilaginous 

 matter in bast-tissue, and the author of this history, it may 

 be remarked in passing, in 1863, and Hanstein in 1864, sug- 

 gested means by which it may be ascertained with certainty 

 that Hartig's sieve-plates are perforated. Meanwhile a number 

 of laticiferous organs had been recognised as forms of vessels 

 in von Mohl's sense, and it was found that such canals are pro- 

 duced by dissolution of the septa of adjacent cells. But the 

 knowledge of the laticiferous organs continued till towards 

 1865 to be very unsettled and defective, and the examination of 

 resin-passages, and the discovery that they are formed by simple 

 parting of cells from one another, belong to modern phytotomy ; 

 Hanstein, Dippel, N. J. C. Miiller, Frank, and others have 

 since 1860 enlarged our knowledge of these forms of tissue. 

 Schacht in 1860 established one of the most important excep- 

 tions to von Mohl's view above-mentioned, by demonstrating the 

 formation and true form of bordered pits in the wood of Coni- 

 fers and in dotted vessels in Angiosperms from the history of 

 their development, and by showing moreover that in all cases 

 where bordered pits are formed on both sides of a partition-wall 

 and the adjacent cells afterwards convey air, there the original 

 very thin partition-wall in the bordered pit disappears, and that 

 consequently in such cases the bordered pits represent so many 

 open holes, through which adjacent cells and vessels com- 

 municate. At the same time another hitherto inexplicable 

 phenomenon received its explanation. Malpighi, and after 

 him the phytotomists at the beginning of the present century 

 had remarked, that the large vessels in the wood are not 

 unfrequently filled with parenchymatous cell-tissue, for the 

 origin of which no one could account. The phenomenon, 



