J'^esseis 



11 



Besides these appearances 

 is, especially in older cells, a 

 cell wall, which appears to be 



Fig. 35. — Two cells from the 

 wood of the Scotch Fir in 

 transverse section, each with 

 a pore,^, widened at its base. 

 By the disappearance of the ori- 

 ginal cell wall the two widen- 

 ings have united to form ±he 

 ' border/ h ; z", intercellular 

 space. 



due to secondary deposit, there 



striation or stratification of the 



due to the varying amount of 



water in the different 



parts of the wall. 



Another important 

 change in the history of 

 the cell is the formation 

 of vessels, or cell fusion. 

 Vessels are formed 

 by the union of several 

 cells (vascular cells), 

 the walls between them 

 being either partially or 

 entirely absorbed. 



Fig. 36.— I. Sieve tube fr6m the White Bryony 

 {^Bryonia dioica), the horizontal partition 

 walls with peculiar thickenings. II. Trans- 

 verse section of a sieve-disc, the upper part 

 represented with the thickening pubstance, 

 which takes the form of wart-like elevations ; 

 the lower part without it. 



Fig. 37.— I. Portion of a scalariform 

 vessel from the Brake {Pteris aqui- 

 lina) : j, j, the transverse division 

 wall broken through in a reticulate 

 manner. II. Pitted vascular cell 

 from the stem of a Grass, Phrag- 

 mites communis^ with numerous 

 small bordered pits. 



"When several cells standing one over another have the walls 

 separating them simply perforated in a sieve-like manner, sieve- 

 tubes or bast-vessels are formed. These are found in various 



