50 



ON THE PLANT-CELL. 



stones of winter pears and quinces ; 

 in the bark of the petiole and stem 

 of Roy a carnosa ; in the stem of Fraxi- 

 nus excelsior (Plate I., fig. 22.) ; and in 

 the fruit stalks of Magnolia (Plate I., 

 fig. 21.), &c. These are called, with 

 peculiar impropriety, branched porous 

 canals. Mohl* was the first who dis- 

 covered this process of growth by de- 

 posit-layers in the cells of the plants, 

 and thus explained one of the most 

 important processes in the life of the 

 plant. 



Formations of the second kind have 

 been longer known, as in the wood 

 of the Yew, where separated spiral 

 fibres and rings are seen, between 

 the windings of which are also large 

 pores. In recent times many simi- 

 lar formations have been observed in the Lime, the Vine, in Primus 

 Padus, Helleborusfatidus, &c. (fig. 34.). Little 

 is at present known of their mode of formation. 

 In the Lime, in the spring, we find in the cam- 

 bium spiral cells with the fibres close together ; 

 in the course of growth these extend, the spires 

 separate from one another, and pores are formed 

 between them, so that here the porous layer is 

 the last formed. How it is formed in other 

 cases is yet a question. 



In the tender spiral cells of the bark of the 

 Asclepiadece and Apocynacece y and in the deli- 

 cate spiral cellular tissue generally, there is 

 sometimes observed the appearance of a crossing 

 of the spiral fibres. This may arise from the 



spires of neighbouring cells, or from the transparency of the walls of the 

 same cell ; but there can be no doubt that in some cases it is an original 

 formation (Plate I., fig. 23.), and arises probably from spires formed in 

 opposite directions. 



21. In many cells the spots remaining free from the secondary 

 deposits become fluid and are resorbed. In this way cavities 

 are formed in the membrane. Upon this depends the distinc- 

 tion between cells and vessels. The last are only rows of cells 

 whose cavities have in this way been brought into union with each 

 other. 



* In his work on the structure of the palm-stem, and in other places. 



33 Porous cells of the petiole of Cy-cas revoluta. Small pores are found where the 

 cells are united to each other, but large ones where the cells open into the intercellular 

 passages. 



34 Vessels from Helleborus faetidus. A, Longitudinal section, a, Two adhering vessels 

 with the pores cut through, and the projections of the spiral fibres, b, The wall of 

 the vessel, without pores touching the wood-cells, with the projections of the fibres. 

 , A vessel seen from without. 



