312 MANUAL OF BOTANY 



nutrition, the protective tissues, and so on. The celJs which 

 conapose such a tissue commonly resemble each other more 

 closely than they do other cells, and are said to obey a common 

 law of growth. They originate generally by division of either 

 a single cell, or of the cells of a well-deiined group or meristem, 

 so that they are held to have a common origin. Only such 

 collections of cells as fulfil these conditions are considered true 

 tissues. 



Meeismatic Tissue, or Meristem. — In the development of 

 the vegetative body of the plant, the new cells are produced by 

 cell-division. In some oases every cell as it is produced possesses 

 and retains the power of division, so that the plant can increase 

 throughout its whole length. This is commonly the case with 

 filamentous plants such as Spirogyra. In most oases, however, 

 the power of cell-division speedily becomes localised' at certain 

 parts of the plant body, which then carry out all farther increase 

 in length. Such points are called growing points : they are 

 generally terminal, and the cells of which they are composed, 

 which have the power of cell- division, are called merismatic cells. 

 The individual cells do not long retain this power, but after a 

 period of growth acquire various forms, and subsequently change 

 but little during life, constituting permanent tissue. Finally they 

 lose their protoplasm, and are no longer living. 



There are two chief types of growing point which are found 

 at the apices of the axis of the plant. In both, multiplication of 

 I cells leads to a continued forward advance of the apex, and the 

 youngest cells are in the front of the mass, so that the growing 

 point is nearly always more or less conical ; two main lines of 

 division of the cells can be seen, one parallel to the surface of 

 the apex, or pericUnal ; the other at right angles to it, or anti- 

 clinal. The merismatic tissue is not of very great extent, and 

 behind it the cells can be observed to be growing or increasing 

 in size, and gradually changing into permanent tissue. 



In the first type the cells which are thus dividing are all 

 alike in appearance, and form a well-defined group which gradu- 

 ally becomes differentiated, as the cells get older, into various- 

 forms characteristic of different regions of the axis {figs. 668 and 

 670). This small-celled meristem is always found in Phanero- 

 gams and in some Cryptogams. 



The other type is marked by the presence at the apex of a 

 single large, generally pyramidal, cell, the base of the pyramid 

 being external {fig. 669). From this, successive segments are cut 

 off parallel to each side in turn, the apical cell growing to its 



