MULTIPLICATION OF CELLS 



237 



388. Bordered pits in 

 pine wood. 



the walls of cells take on certain definite markings. Some 

 of the names applied to these markings are: 



Pitted, with little holes or depressions, forming very 

 thin places, as seen in seeds of sun- 

 flower, and in the large vessels in 

 the stem of the cucumber. 

 Bordered pits, when the pits are in- 

 closed in the cell-wall, as in wood of 

 pines and other conifers. Fig. 388. 

 Spiral, with the thickening in a spiral 



band, as in the primary wood of most woody plants 

 and in the veins of leaves. Fig. 389. 

 Annular, with thickening in the form of rings; seen 

 in the small vessels of the bundles in stem of Indian 

 corn. Fig. 389. 

 Scalariform, with elongated thin places in the wall, 

 alternating with the thick ridges which appear like 

 the rounds of a ladder. Fig. 389. These are well 

 shown in a longitudinal section of the root of the 

 brake fern (Pteris). 

 391. MULTIPLICATION OF CELLS.— Cells give rise to 

 new cells. Thus does the plant grow. The most com- 

 mon method by which cells are multiplied is that called 

 cell division. A modified form of cell 

 division is called budding. Cell di- 

 vision is a process by which two 

 (or more) cells are made from one 

 original cell. Cells which have an 

 abiDidance of protoplasm are usually 

 most active in cell division. The 

 process is at first an internal one. 

 The nucleus gradually divides into two masses and 

 the protoplasm of the cell is apportioned between 

 these two nuclei; a new cell -membrane, or partition 

 wall, is usually thrown across and the cell is completely 



a. Markings in cell-walls. 

 sp, spiral; an, annular; 

 «c, scalariform. 



