CHAP, in.] of Cell-membrane in Plants. 281 



that there is a sheath to the whole bundle composed of strongly 

 thickened fibrous cells, that each of these cells has its own mem- 

 brane and is entirely closed, and that they resemble the bast 

 and the fibrous elements of the wood of Dicotyledons. The 

 segmented wood-cells and the parenchyma-cells of the wood 

 arranged in rows are incidentally noticed. Under the name of 

 fibrous tubes he included the cells of the sclerenchyma-sheath 

 of many vascular bundles and the true bast and wood-fibres, 

 which latter he says are wanting in the Coniferae. He explained 

 the secondary growth in thickness of rind and bast by the ex- 

 ample of the shoot of the vine, in which he correctly distinguished 

 the medullary sheath and the spiral vessels. In herbaceous 

 Dicotyledons he found the bundles of vessels to consist of a 

 bast portion and a woody portion, and he attributed the forma- 

 tion of the compact wood of true woody plants to the blending 

 together of the woody portions of these separate bundles. 



In discussing the parenchymatous cell-tissue he rejects em- 

 phatically and on good grounds the origin of new cells from 

 the granular contents of older ones, which had been the view 

 of Sprengel and Treviranus, as also the theory of Wolff and 

 Mirbel, while he maintains against Mirbel especially, that the 

 separation of fibrous tubes is possible even where no dividing 

 line can be seen between them in the cross section. He con- 

 siders that both in thin-walled and thick-walled parenchyma the 

 dividing wall is double and the cell-membrane entirely closed. 

 'It appears,' he continues on p. 86, 'from these observations that 

 cellular substance consists of separate closed tubes, which may be 

 round or oval, or more or less elongated, or almost cylindrical 

 in shape, and these by mutual pressure assume an angular and 

 flattened form, which is either regular like the cells of the comb 

 of bees or more or less irregular. Such an aggregate of sepa- 

 rate cells (and here he is certainly quite right) has nothing in 

 common with a tissue, and the word cell-tissue seems there- 

 fore less suitable than the term cellular substance, composed of 

 cell-like tubes.' Further on he rejects Mirbel's idea of the 



