FORM OF THE PLANT-CELL. 



A 49 



61 



The wood of the species ofMammillaria differs little from this. At first 

 sight it appears to consist of an entirely uniform tissue of somewhat ex- 

 tended cylindrical cells, which are distinguished by a most delicate spiral 

 band projecting far into the cell (fig. 50. B). By greater observation 

 upon longitudinal and transverse sections of the cells in which the spiral 



50 



fibres project less into the cells, it will be found that they are in com- 

 munication with one another, and allow air to pass through (fig. 50. A a, 

 B a). This is the simplest form of the so-called vessel. 



In another way a simple kind of wood is formed in Carolinea minor. 

 It is extremely light and soft (like cork), and consists of regular paren- 

 chyma-cells, slightly elongated and somewhat porous, and of individual 

 rows of much broader and longer, cylindrical, and clearly porous cells 

 (called vessels), standing in open communication with one another. Very 

 similar to this is the wood of Bombax pentandra (figs. 51, 52.), where 

 we find, between the parenchymatous cells, individual, long, but tolerably 

 thin-walled, prosenchyma-cells (figs. 51, 52. b). From this to the ordinary 

 wood a transition is formed by some wood in my possession from the 



49 A, Transverse section of the wood of Cycas revolvta. B, Longitudinal section of 

 the same, parallel to the medullary rays, a, Tn both figures, medullary ray cells. The 

 most elongated cells have upon their walls innumerable large pores. 



50 A, Transverse section of the wood of Mammillaria quadrispina. B, Longitudinal 

 section of the same, parallel to the bark, a and c, Spirally formed plates inside the 

 cells : these cells contain only air. 6, Medullary ray cells. 



