FORM OF THE PLANT-CELL. 43 



Where they are perfectly invisible, the application of a little iodine will 

 frequently make traces of them evident : spiral formations are also fre- 

 quently observed, for the first time, when the vessels begin to carry air, 

 on account of the different relation of the air and the solid substance of 

 the vessel to light. 



In addition, I find that in all spiral formations the spires are narrower 

 the younger they are, and also more simple and unbranched in their 

 structure : in the most abnormal forms for instance, the annular ves- 

 sels I have found most evidence of this.* From these facts, then, it is to 

 be inferred,, that the foundation of all the forms of spiral formation are 

 simple, unbranched, narrow, spiral fibres lying one upon another ; and 

 upon this simple hypothesis, combined with the undoubted fact that all 

 spiral formations become first known to us after they have been present 

 for a long time, and during this period have become considerably 

 changed, we may easily explain all the phenomena observed. 



It must, however, be confessed, that we are still far from being able to 

 supply a rational induction for the phenomena of spiral formation : we 

 have, even now, scarcely any indication of the relation which exists 

 between the formation of a spiral and the nature of the plant or of the 

 cell in which it occurs. We should gain a secure point for the develop- 

 ment of the whole doctrine if we could find a single case, even a case 

 which was not connected with the formation of a spiral fibre, in which 

 we could prove, from the existence of a vegetable cell under certain cir- 

 cumstances, that a tendency to a spiral direction must necessarily follow. 

 Could we trace the spiral direction of the circulation in the central cell 

 of Chara to a peculiarity in the nature of the cell as a necessary con- 

 sequence, then we should obtain for all spiral phenomena an entirely 

 different signification. In looking at the peculiar brown-coloured spiral 

 fibres in the cells of the sporidia of the Jungermannice, the movable 

 spiral fibres in the antheridia of Charas, Mosses, Jungermannice, and 

 Ferns, it appears to be not improbable that we include under the name 

 Spiral Fibres very different things. In these instances, the one is a 

 peculiar form of the deposit layer of cell-membrane, a non-azotised sub- 

 stance (cellulose) ; and in the other, an especial form of the mucus, the 

 nitrogenous constituent of the cell. 



Forms of the Spiral. After the spire has been developed in a cell, the 

 latter often becomes extended. 



A. This extension produces the following modifications : 



a. When the turns of a single fibre early grow to- 

 gether so as to form a ring, whilst the free fibres become 

 torn or resorbed, so that the cell exhibits either the 

 rings alone, or mixed with single turnings of the spire, 

 these rings are ordinarily little or scarcely at all con- 

 nected with the cell- wall (Cellules annuliferce) (fig. 18.). 



This occurrence has been observed in its entire 

 course in the stem of many Tradescantia, in the root- 

 stock of Equisetum arvense, and in some other plants. 

 In many other plants I have failed in detecting the 

 first formation of the rings. In the Cactacece, in which 

 the rings are so large, I have not been able to observe their formation. 



* Notwithstanding Mohl's objections (Flora, v, 1839, Nos. 43,44.), I still, after 

 repeating my observations, hold the view I took at first. (Ibid. Nos. 21, 22.) 



18 A fibrous cell, with two single rings, from the neighbourhood of the woody bundles 

 of the Opnntid peruviana. 



