ORIGINS OF AGAMIC PATTERNS 



641 



cated by the data at hand. The early aggregate, such as in Figure 204, A, 

 is apparently not essentially different in pattern from an adventitious 

 plant bud, but its further development depends on addition rather than 

 on growth and division of cells. 



THE PSEUDOTHALLI OF OTHER FORMS 



Certain species of diatoms give rise to axiate systems resembling the 

 thalli of algae but consisting of diatoms, apparently not even in contact 

 but imbedded in a firm jelly-like substrate secreted by them. The growth 

 form of one of these diatomaceous pseudo thalli is shown in Figure 205. 

 The branches are flat and thin in the plane of the paper. Different 

 branches of a single pseudo thallus are approximately equal in width; and, 



Fig. 205. — Part of a diatomaceous pseudothallus 



as it attains a certain length, each branch undergoes dichotomous divi- 

 sion. This pseudothallus consists of diatoms, more or less regularly ar- 

 ranged in longitudinal rows in the jelly-like secretion, their long axes coin- 

 ciding in direction with the rows and the axiate pattern. This definite, 

 orderly axiate pattern must result from definite, orderly relations between 

 the diatoms giving rise to it. If all grew and divided at the same rate, 

 such a form could not arise. Evidently the chief, or only, region of growth 

 and division is apical; but the individual diatoms of the apical region 

 must be integrated in some way into a system resembling, in certain re- 

 spects, the growing tips of various algae, in which very similar thallus 

 patterns appear. Differential susceptibility and differential reduction in- 

 dicate a gradient in these pseudothalli with high end apical, and extending 

 for at least a short distance from the apical ends of the branches (Child, 

 19 1 9/). Some other diatoms give rise to branching systems of more or 



