i6 



Elementary Botany 



strong solution of the same substance, it enlarges by a pfOCegg 



of accretion — that is, a series of fresh layers form upon its exterior. 



I. II. III. 



Fig. 17.— I. Spherical cell from the flesh of the Peach. II. Ellipsoidal cell from the 

 flesh of the Peach. III. Hour-glass-shaped cell from the flesh of the Peach. 



Growth by intussusception often 

 produces a change in the shape of 

 the cell. The normal form is more 

 or less spherical ; this, however, is 

 rarely preserved except in the case 

 of unicellular plants. If the wall 

 grows more vigorously at the two 

 extremities than at the sides, the 

 cell will become oval or elliptical 

 (fig. 17), then elongated (fig. 19), and 

 lastly fusiform or spindle-shaped 

 (fig. 18). 



On the other hand, if a very 

 vigorous growth takes place at cer- 

 tain parts whilst the rest of the 

 cell wall is but slightly developed, 

 several protuberances will be pro- 

 duced upon the surface, and a stel- 

 late or star-shaped cell is formed 

 (fig. 20). 



Another cause modifying the 

 shape of individual cells is to be 

 found in their mutual pressure the one upon the other during 



Fig. 18. — I. Fusiform cell from the 

 wood of the Spruce Fir, with bor- 

 dered pits. II. Fusiform bast- 

 fibre of^ the Larch. 



