170 University of California Publications in Zoology. [Vol.6 



creases with each successive whorl, the increment decreasing so 

 thai the leaf number in the later whorls is practically constant. 

 Our curve of the size of the zooids takes the same course as the 

 curve for variation in leaf-number until the mean length or width 

 becomes fairly constant, when it suddenly drops. Observation 

 shows that in leaf -size also there is a decrease at each end of the 

 branch. 



The question suggests itself — Did Dr. Pearl take all of the 

 whorls into account, or would more whorls have been produced 

 at the tip of the branches, and if so. would their leaf number 

 remain constant for a time and then toward the end of the life- 

 time of the plant, vary again and grow smaller? 



If he had taken .s/>r of leaflets rather than number, the graph 

 would probably have been like the graph for the length of zooids, 

 the maximum occurring toward but not at the base where the 

 oldest whorls were found. 



The two results for leaf number and size of zooids might pos- 

 sibly be brought into harmony by further observation of the size 

 curves. From a study of I he graphs of the block (figs. 10 and 

 1 1 ) . it seems likely that the maximum value shifts to the proximal 

 portion of the chain as the zooids grow; in fact the graph for the 

 oldest block is almost the reverse of that for the younger blocks, 

 Would this graph, if the zooids were all full grown, show a 

 gradual approach to a constant size as one approaches the 

 proximal end of the block and would the most proximal zooid 

 of all. once the smallest, catch up with those near it ? The nature 

 of the material makes it unlikely that we will be able to deter- 

 mine this definitely; since, aside from the fact that chains of this 

 age break apart so easily, the distal zooids are increasing in width 

 so rapidly that length alone is not an accurate method of indi- 

 cating size. 



"With the leaves of a branch, we see that those at the tip 

 seldom or never catch up in size with those at the base. In almost 

 every instance of periodicity in growth in the plant world we 

 find that this condition holds; that smaller members of a series 

 are a1 the beginning and at the end of the period of growth while 

 the maximum ones are in between. We conclude then, that the 

 curves from the serial measurements of the blocks of zooids of 



