450 A TEXTBOOK OF THEORETICAL BOTANY 



the meristematic character, so that meristem, in addition to forming per- 

 manent tissues, is also self-perpetuating. In most Pteridophyta the function 

 of producing new cells is localized in a single apical cell instead of being 

 distributed through a meristematic tissue. In this they resemble certain 

 Thallophyta. 



The tip of a stem is usually occupied by a bud, which is composed of 

 immature leaves produced by the growing point itself. The latter lies in the 

 centre of the bud, and a median longitudinal section will show that it is in 

 fact the actual apex of the stem, which tapers rapidly within the bud, to end 

 at this point. The meristem of the growing point is extremely soft, delicate 

 and juicy, as seen with the naked eye. A similar tissue occupies the tip of 

 the root, but no bud surrounds it in this case, because roots have no leaves. 



Microscopically the cells of the growing point are small, fairly regular in 

 size and shape and have thin, delicate cell walls formed of a mixture of 

 cellulose and protein, with an intercellular layer of mucilaginous pectin. 

 They have prominent spherical nuclei, which are very large relative to the 

 size of the cells, and the cytoplasm is dense and completely fills the cells. 

 When a meristem cell divides, each half quickly grows to the size of the 

 original cell, thus doubling the amount of protoplasm. It follows that it is 

 here that the chief synthesis of protoplasm goes on. The cells fit closely 

 together, without intercellular spaces, and their shapes are those of plastic 

 spheres in mutual compression, due to their active growth. They are either 

 dodecahedra (twelve-sided) or tetrakaidecahedra (fourteen-sided). The 

 youngest cells have no vacuoles and they never possess chloroplasts, though 

 transitory starch grains may sometimes appear. 



Opinion varies as to the exact limits of the term meristem. Some confine 

 it entirely to the non-vacuolated cells, but there is a considerable interval 

 between the first vacuolization of the cell and its completed differentiation, 

 and many prefer to extend the term generally to all those cells which continue 

 to divide, even when somewhat altered from their youngest form. 



The meristem of the growing point is called the apical meristem, and 

 the term primary meristem is applied both to it and to all meristem tissues 

 derived directly from it. Thus the cambium in Dicotyledons and the 

 meristems of lateral shoots are also usually primary meristems. In many 

 Monocotyledons there are intercalary meristems at the bases of leaves, and 

 similar embryonic regions occur in older parts of Dicotyledons. All these 

 are primary meristems. True secondary meristems, i.e., those formed 

 from tissues previously differentiated, are rare, the chief example being the 

 cork cambium (see Chapter XXI). 



When we consider that the development of the meristem cells is responsible 

 for determining the specific form of the plant and the structural anatomy of 

 the older portions, it is obvious that it is the centre of many activities of great 

 interest, about which we know very little. The evidence seems to show, 

 however, that almost without exception the divisions of meristem cells are 

 equational, and therefore that the potentialities of the cells produced are 

 equal. There is little sign in plants of the primary segregation of germ cells 



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