306 BOTANY PART i 



differentiation with the distinction of apex and base is met with also 

 in more simply-constructed plants. While cases have been already 

 referred to in which this distinction is determined by an external 

 factor, in all higher plants the polarity is specific and depends on 

 internal causes. We can neither cause growth with polarity in 

 a spherical apolar Alga, nor induce a higher plant that possesses 

 polarity to become apolar. 



The polarity once it has been defined in the egg- cell is on the 

 whole maintained throughout growth. In some plants, however, it 

 can be seen to be altered from internal causes. 



Thus in species of Platycerium and Adiantum among the Ferns and in 

 Neottia nidus avis among the Orchids, shoots are formed directly from the growing 

 points of roots. In the Adder's-tongue Fern (Ophioglossum] the vegetative repro- 

 duction depends entirely on the formation of buds close to the growing points of 

 the roots. The apex of some fern leaves also (e.g. Adiantum Edgeworthii] may 

 grow directly into a shoot. 



Symmetry. Every growing point effects in a characteristic fashion 

 the further construction of the organ to which it belongs, and also 

 provides the primordia of lateral organs, the distribution of which as 

 they appear is definitely determined, and may be radial, bilateral, or 

 dorsiventral. Thus a certain symmetry already exists in tjie growing 

 point, and, at least in many cases, is determined by purely internal 

 causes ; in others external factors have a preponderating effect. 



B. Growth and Cell Division 



Growth, once started, does not always proceed uniformly. Some 

 Algae such as Vaucheria or Fungi like Saprolegnia continue to extend 

 the cell by apical growth. In the great majority of cases, however, 

 there is a limit to this, and when a certain size has been exceeded 

 the normal mass of the cell is regained by division. There is no 

 regular rule, since the process depends not only on external conditions 

 but in great part on internal. Thus, for example, divisions proceed 

 rapidly at the growing point while they become less frequent later, 

 though growth still proceeds. In accordance with this the size of 

 the cells as a rule increases considerably on passing from the growing 

 point to the region composed of permanent tissue behind. The 

 volume of the nuclear mass is also of importance in the question 

 of the size of the cell. It has been possible in some instances to 

 obtain a nuclear mass twice or four times that of the normal nucleus 

 in a cell ; all the cells derived from such a cell proved to be consider- 

 ably above the normal in size ( 70 ). 



A mean volume or mass of the cell can always be regarded as one 

 of the hereditary characters of a species. When species of different 

 sizes are compared, the range in size of cell is not found to be as great 

 as that in the size of the plant as a whole. In other words, large 



