2 2 PATTERNS AND PROBLEMS OF DEVELOPMENT 



MULTIAXIATE GROWTH FORMS 



The branching or multiaxiate plant, whether alga or tree, originates as a 

 series of buds from the primary axis. Each branch of the common type 

 of multiaxiate plant consists, at least during its development, of a vegeta- 

 tive tip, which is the first part formed, and other axial levels formed by the 

 tip. Protoplasmic growth decreases basipetally in the tip, except as new 

 bud loci are activated in certain spatial relations to the tip; and other 

 forms of growth, such as increase of cell size by increase in size of vacuole, 

 usually also decrease basipetally from a certain level below the tip where 

 they begin. Cells in certain regions of these lower levels, however, may 

 still continue division and protoplasmic growth and bring about increase 

 in transverse diameter along the axis. It is evident that the growth form 

 of the whole multiaxiate plant must be determined by relations between 

 the different axes. The tapering growth form of the fir and many other 

 conifers, of various other trees and herbaceous plants, and of many algae 

 with branches radially arranged or in a single plane must result from rela- 

 tions of some sort between the main axis and lateral branches. Moreover, 

 the variations in this growth form, that is, from a short cone with large 

 base to a long cone with small base, and the almost cylindrical, ovoid, or 

 nearly spherical forms of some other trees can result only from orderly, 

 graded differences in growth of the different axes, differing in different 

 species. Many multiaxiate plants — many trees, for example — show, when 

 young, a more or less conical form with marked difference in growth and 

 form of the main axis and lateral branches; but later these differences be- 

 tween main and lateral axes may decrease, and all may become more or 

 less completely equivalent and a form with spreading crown of more or 

 less similar branches results. The quantitative study of growth of lemon 

 shoots by Reed (1928) shows differences in growth of apical and lateral 

 shoots of definite and orderly character. Plants with a multiaxiate in- 

 florescence usually show a gradient, either acropetal or basipetal, in de- 

 velopment of flowers. In the cotton plant, for example, blooming begins 

 at the bases of the lowest fruiting branches and progresses acropetally 

 along each branch and along the main axis (McClelland, 191 6). Many 

 other examples of both acropetal and basipetal progress of blooming 

 might be given. Which of the two orders occurs apparently depends on 

 the condition of the tip at the time of flowering. If it is still growing, the 

 order of flowering is, in general, acropetal; if early transformation of the 

 tips into flower axes occurs, the order is basipetal. 



The growth form of the multiaxiate ciliate protozoan Zoothamnium is 



