64 



BOTANY 



PART I 



from their cells ; these swarm-spores (Fig. 97 A) serve to multiply 

 the plant vegetatively. They soon settle down, form a thin cell 

 membrane on the surface of the protoplast, and proceed to give rise 

 to a filament. In more highly organised plants the ovum, from which 

 the development starts, has no cell wall until it has been fertilised ; 

 from this stage on, all the cells composing the plant are surrounded by 

 cell walls. At the growing points of plants the cells are separated 

 from one another only by extremely thin membranes or cell walls. 

 As the cells increase in number by repeated division, new cell walls 

 are being continually introduced between the existing ones. The 



FIG. t>G.--Htrongly thickened cell 

 from the pith of Cli'indtix rHnllm. 

 , . Middle lamella ; i, intercellular 

 space; t, pit; ie, pitted} cell wall 

 in surface view. (X 300.) 



l-'ic. lii. Part, of asclerenchj- 

 inatous libre IKnii Vln<: 

 iiiftjm: The striationsof the 

 outer layer* are in<>re appar- 

 ent than those of the inner 

 layers. The thicknessof the 

 wall, as seen in optical sec- 

 tion, is also .shown. (X500.) 



rapid growth in length which sets in a short distance from the 

 growing point, as a result of the increase in the size of the cells, must 

 be accompanied by a corresponding GROWTH IN SURFACE of the cell 

 walls. So long as this growth in surface continues, the cell walls 

 remain thin. After the cells have attained their ultimate size, the 

 GROWTH IN THICKNESS of the cell walls then begins. The growth in 

 surface of the cell wall may either involve the introduction of new 

 material, or may take place without this. In the latter case the mem- 

 brane would become thinner if new lamellae were not simultaneously 

 applied to its surface. Growth of the wall by the introduction of 

 new particles between those previously existing is termed GROWTH BY 

 INTUSSUSCEPTION, while that which occurs by the laying down of new 

 lamellae on the surface of the older ones is called GROWTH BY APPOSI- 



