DIV. II 



PHYSIOLOGY 



307 



plants are mainly (but not entirely) determined by a large number of 

 cells ( T1 ). 



Little is known as to the particular causes of cell division ( 72 ). 

 It doubtless depends on a very complicated succession of phenomena ; 

 these concern not only the protoplasm but the nucleus which initiates 

 the process. In the growing point of the shoot in higher plants, and 

 also in some Algae, a certain periodicity is evident in the cell division 

 which occurs more frequently at night than during the day ; it is 

 evident that light has an inhibiting effect, but unknown external and 

 internal factors must co-operate. Not merely the fact that a new 

 cell wall is formed but the direction in which it arises is a problem 

 of developmental physiology. It has long been observed that the 

 position of the new cell walls 

 shows a striking similarity to 

 the behaviour of weightless 

 liquid films such as those of 

 soap bubbles. The latter tend 

 to contract to the least possible 

 surface, and therefore are in- 

 serted as nearly as possible at 

 right angles on the walls already 

 present. In spite of the great 

 similarity between the arrange- 

 ment of cell walls on the one 



, , , . ,. . . . , FIG. 266. Diagrammatic representation of a growing 



hand and of surfaces of minimal po^t. (After SACHS.) 



area on the other, it would be 



unsafe to conclude that the same causes determine the position in 



the two cases, for the cell wall is never fluid. 



The principle of the rectangular intersection of cell walls is strikingly shown 

 in the growing points of phanerogamic plants. In these, as is shown in SACHS' 

 diagram (Fig. 266), the cell walls form two systems of parabolas which have a 

 common focus and intersect at right angles. The one system (Fig. 266 I-VI] runs 

 more or less parallel to the surface of the growing point ; these cell walls are 

 termed PERICLINAL. The walls at right angles to these (1-11 ) are termed 



ANTICLINAL. 



C. Further Periodic Changes in Vegetative Form 



Other periodic phenomena often occur while growth is active, 

 There are, for example, periodic changes in the form of the leaves and 

 stem, which are not only quantitative but qualitative ; foliage leaves 

 alternate with scale leaves or bracts, or leafy shoots with rhizomes, 

 the transitions being either gradual or abrupt. The correlation of 

 growth, already considered (p. 296), is concerned in these phenomena. 

 The existence, or rather the activity, of a certain quantity of foliage 

 exerts an influence on the primordia forming at the growing point 



