42 



ON THE PLANT-CELL. 



17 



The individual spires of fibres, or particular spots of the spires, 

 often grow together. From these circumstances a very varied 

 configuration of the cell-wall results, which may be comprehended 

 under two divisions. First, where the fibres are clearly separable 

 (fibrous cells, cellules Jibrosce) ; and, second, where the fibres are so 

 grown together, that they appear like a continuous membrane 

 beset with little pores (porous cells, cellules porosae). 



Nature and Origin of the Spiral. A spiral may be formed either 

 from left to right or from right to left. If we take the distance from the 



commencement of the one spiral to 

 the commencement of another di- 

 rectly above it, and make it a, then 

 + a will be the expression of the 

 spiral winding to the right, and 

 a that of the spiral to the left, 

 and a a the expression of the 

 ring passing between them. The 

 right spiral is the most frequent, 

 but the left occurs often enough 

 to render the resulting ring as 

 indifferently the produce of either. 

 It is, however, possible for the 



ring to have another origin. Each spiral may be divided into two 

 halves, which, regarded from the same point, proceed in different direc- 

 tions. If the one half of the winding be from right to left, the other 

 must be from left to right. Two spirals proceeding in the same direction 

 would run parallel, but would cross each other on a flat surface (fig. 17. c). 

 Two spirals running in opposite directions would cross each other twice 

 in the course of one entire turn (fig. 17. d). This last form has not yet been 

 observed. Link* thinks he has seen it, but his own drawing shows that 

 it belongs to the first case. The lines crossing each other in the walls 

 of the liber cells of Apocynacece, may be explained upon the supposition 

 of there being two layers, lying one upon another, turning in opposite 

 directions. 



It is easy to perceive in the larger spiral fibres, when they are cut 

 transversely, that they are homogeneous. In the old fibres of Arundo 

 Donax they consist of a fibre lying on the wall, and one on each of the 

 three free sides. In a transverse section, under a good microscope, it 

 may be easily seen that the fibres are never round, but that they consist 

 of a flat, thicker or thinner, band, whose free edge or side is, perhaps, at 

 the utmost a little rounded. (Plate I. figs. 18, 19, 20.) The idea that 

 they are hollow canals arises out of defective observations. 



I do not believe that the first origin of the spiral has been observed. 

 It appears to me that it exists much earlier than can be at present 

 detected by our optical instruments, as it is formed from a matter which 

 cannot be distinguished from the contents on the walls of the cell. The 

 spires are more transparent in their earliest stages, and when invisible 

 as a whole they may be observed as small projections upon the edges of 

 the cell : in this state they are contracted, and are thus more bulky. 



* Elementa Philosophise Botanicee, p. 167. 



17 Diagram, a, Left spiral, b, Right spiral, c, Two left spirals in one cell, d, Two 

 spirals right, and two left, in one cell. 



