88 SCALARIFORM TISSUE. [BOOK i. 



opposite the intercellular and intervascular spaces the pressure 

 is much less ; thus the fibre within the compressed spiral is 

 bent into as many sides as there are surrounding and pressing 

 vesicles or vessels. Accordingly, if the spiral fibre be examined 

 at this period, it will be found bent as above mentioned, and 

 the natural curve of the fibre straightened. Opposite the 

 intercellular or intervascular spaces, i. e. at the bendings of 

 the fibres, the latter become firmly adherent to the membrane, 

 thickened and united to the fibres above and below. These 

 thickened portions form the line of space running between 

 the rows of dots. The dots themselves are formed by the 

 spaces left between the portions of the fibres opposite the 

 convexity of the surrounding cells and vessels. Thus, when 

 we examine the tubes at this stage, we find the membrane 

 and fibre united so firmly, that they are with great difficulty 

 separated. The vegetable substance which fills up the inter- 

 cellular spaces often also becomes firmly adherent to the 

 membrane and fibre, so that, when we dissect out one of these 

 tubes from the surrounding parts, we often find the remains 

 of adherent portions which existed opposite the intercellular 

 spaces; therefore the number and arrangement of the dots 

 must depend entirely upon the surrounding vessels and cells. 

 When the compressing and compressed tubes are equal in 

 size, the dots extend nearly across the face or opposed side of 

 the tube ; and when several small tubes and vessels compress 

 a spiral, so as to convert it into a dotted tube, the dots will 

 be small and numerous. The observation of Schleiden, that 

 ' in consequence of the deposition of formative substance., 

 the pore appears the rounder the more the cell is developed/ 

 is, I think, incorrect; I believe the reverse to be correct. 

 The examination of the young and old stems of any plant 

 containing these vessels will prove this. If the fibre be 

 separated from the surrounding parts in the early stages, it 

 will be found bent and thickened at the bendings ; and often- 

 times we can find portions of membrane, &c. adhering as above 

 mentioned. These vessels are generally observed in plants 

 whose growth is rapid, so that in older stems we cannot 

 expect to find the arrangement persistent; but in a large 

 number of plants it can readily be perceived, especially where 



