350 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
head. He continued in this way to the end of the flowering season 
and got a decrease in flower number in successive ‘‘bud-genera- 
tions." The only fact revealed that clearly bears on the question 
of flower number in relation to position is that there appears to be 
a higher flower number for terminal heads than for those borne 
on side-branches. 
Schüepp (’13) has made detailed statistical studies on 
Aconitum Napellus. One of his chapters is devoted to variations 
within the individual (partial variability) and his data show that 
to a certain extent quantitative characters are functions of the 
position of the organ on the plant. This is very apparent for a 
character like leaves, which in the vegetative parts are petioled, 
large, and have 40-50 points, while in the reproductive regions 
they are sessile, small, and one-pointed. He also gives the number 
of perianth parts, nectaries, stamens, and carpels for three regions, 
base, middle, and top of plant, and in all finds a slight decrease in 
the number of parts from the base upward. 
Klebs (7о6) showed that there were slight differences between 
lateral and terminal inflorescences in Sempervivum. Vogler 
(12), whose work has been discussed earlier, presents data to show 
` differences according to position between number of flowers in the 
inflorescences of Umbelliferae and Compositae. 
Such differences as have been noted have a bearing on the 
much larger question of the periodicity shown in the development 
of an individual plant. Braun, Sachs, van Tieghem, J. W. von 
Mohl, and de Vries have contributed much to this question. 
Tammes (03) reviews the literature on this subject and gives to 
von Mohl the credit of establishing the fact that there is a period- 
icity in cell division, so that the longer internodes have more as 
well as longer cells than the shorter. Tammes (’03) investigated 
a large number of plants and showed that there is a periodicity 
in development for length and breadth of leaves, length of petioles, 
and number of main veins. 
These studies show that partial variability in respect to the 
number of parts in a complex structure such as a flower or a 
flower head is to some degree related to position on a plant in- 
volving time of development, and therefore introduces an element 
of differentiation. This places an emphasis on processes of devel- 
