THE VEGETABLE CELL. 



91 



Fir/. 33. 



secondary layers are deposited in the form of a net-wurk, or of parallel 

 cross fibres on tlie transverse or oblique partitions of the vascular utricles, 

 while the primary membrane is regularly absorbed between these fibres, 

 so that the open communication between the vascular utricles is not 

 interrupted. 



Ohserv. 3. In the description of the structiu^e of the cells and vessels, 

 I have mentioned the spiral and reticulated coui*se of the fibres as two 

 distinct modifications of the structure of the secondary membrane. Since 

 ti'ansitions between the two structures frequently occur (fig. 33), and 

 since when the fibre is reticulated the pits are arranged more or less dis- 

 tinctly in spiral lines ; since, moreover, the pits scattered over an uniform 

 membrane frequently have a lungish form, and their long diameter like- 

 wise situated in an oblique spiral direction, the thought readily presents 

 itself that spiral structures form the basis of secondary membranes of all 

 cells and vessels, and that the other forms owe their origin to subsequent 

 transformation of the spiral cell and spiral vessel. The view has been 



expressed by most phytotomists in reference to the 

 vessels ; but the conceptions that have been formed 

 of the processes occurring in this metamorphosis 

 were for the most part of rather a rough character. 

 Thus the notion was extensively embraced, that the 

 spiral fibre could not follow the expansion which 

 the vessel underwent during its growth, and tore up 

 into fiagments, wiiich again united into rings, and 

 thus brought about the formation of annular vessels. 

 Completely as this idea, which was a contradiction 

 to all observation, had been refuted by Molden- 

 hawer, it remained a standing article in all phy- 

 totomical writings up to " MeyerCs Fhydologie,^'' 



Schleidon (" On the Spiral Structures in the Vege- 

 table Cell" Flora^ 1839) sought to explain the origin 

 of the annular vessels from the spiral vessels in a 

 manner less easy to refute, assuming that in each 

 case two turns of a spiral fibre grew together into a 

 ring, while the rest of the fibre, running between 

 ^^^iSi^sMk^S^li^S *^^ these rings was subsequently dissolved. My own 

 hyindiim, observations ("(9?i the Structure of the Annular Yes- 



sekf^' in my " Vermischte Schrifien^^ 285) compel me 

 to declare most decidedly against this explanation, since they demonstrated 

 the rings to be primaeval, original structui-es, from their very first appear- 

 ance, and the seeming transitional stages from spiral vessels into animlar 

 vessels to be permanent intermediate forms between the two kinds of 

 vessels. 



The idea that the reticulated vessels are produced from spiral vessels 

 has been more extensively defended, and especially lately by Schleiden 

 and linger (Linmmob, 1841, 394y). Nothing appeared simpler than the 

 assumption that cross fibres were formed between the convolutions of the 

 spiral fibre, and that the spiral was thus converted into a reticulated 

 vessel. But two circumstances lead me to reject this notion most de- 

 cidedly. In the first place, observation of the vessels in which the second- 

 ary layers have just begun to be formed^ gives evidence that the delicate 



