THE TISSUES OF THE STEM 



59 



cell for storage and other functions, and the fibrous cell for mechanical 

 resistance. The forms of these various products of the cambial 

 cells, as seen in the stem of the Lime, are shown all to the same 



scale in Fig. 38, i.-x. 



When a large vessel of the wood is being formed, cambial cells in 

 a longitudinal row widen greatly, pushing aside the adjoining cells. 

 The lateral walls become thickened, and usually pitted, but the end 

 walls are absorbed, and the protoplasm disappears. Thus they 



v/. 



/v 



I 



F.o.S 



Mm. 



Fig. 38. 



Cambium cell of the Lime, and its various products in the secondary wood and 

 secondary bast, all drawn to the same scale, and seen in tangential section. 1.- 

 nucleated cambium cell. ii. = fibrous tracheid. iii. = group of cells of wood- 

 parenchyma, iv. v.=single lengths of vessels, the oblique terminal walls having 

 been absorbed, vi. vii. = wood fibres, bent to save space in the figure, via. = group 

 of bast-parenchyma. ix. = single length of sieve-tube, with oblique terminal walls 

 perforated. x.=bast fibre bent to save space in the figure. ( x 75) 



become tubes for transit of the sap that fills them. As they have lost 

 their protoplasm they are dead elements. Smaller vessels develop 

 similarly, but with less disturbance of the neighbouring cells (iv. v.). 

 When wood-fibres are being formed the cambial cells elongate, and their 

 pointed ends bore their way upwards and downwards, with a sliding 

 readjustment among themselves. As considerable tracts of cells may 

 develop thus alike, and as the cells themselves take a more or less 

 sinuous course, they become interlocked, almost like the strands of 

 a rope. At the same time their walls become greatly thickened, and 

 woody, and their protoplasm disappears. Their function is thus not 

 vital but mechanical (vi. vii.). When wood-parenchyma is being formed 



