244 BOTANICAL GAZETTE [SEPTEMBER 
NOTES FOR SLUDENTS 
Periodicity in tropical trees——KLrEBs” has added some important evidence 
on the much disputed question of the cause of periodicity in tropical trees. 
It has been observed that some tropical trees in the very nearly constant 
climate of their native habitat show no vegetative periodicity, while most of 
them show an alteration of growth and rest in some form or other. VOLKENS 
and others attribute this behavior or difference in behavior to hereditary 
characters; while Kress thinks of it as the result of external (nutrient) 
conditions acting upon the specific structure (hereditary characters) of the 
plants. KLEss would say that the course of development of any plant or plant 
primordium is determined by two things, specific structure of the unit and the 
condition under which it develops. Every such unit has many potentialities 
so far as course of development is concerned; the one realized depends upon 
the condition under which it develops. The reviewer feels that the dual nature 
of the determiners of the course of development of the individual should sink 
deeply into the minds of all botanists. It will clear away much foggy thinking 
frequently manifested by such statements as “this character is due to heredity” 
and “that one to environment.” Every character is due to both. Here the 
ausidaive philosopher’s attempt to get at the one ett factor fails, for 
there are two 
The work was done in the greenhouse at Heidelberg on the following 
forms in the main: Terminalia Catappa, Theobroma Cacao, Albizzia stipulata, 
and Pithecolobium Saman. Kes states that since growth depends upon a 
great number of factors, rest can be procured by any one of these sinking to a 
certain minimum, and this minimum varies greatly for the different species. 
Kurs worked especially with light and nutrient salts. The light intensity re 
its daily durations at Heidelberg gradually fall to a low minimum late 
December. With good salt supply there were 3 types of response t ich 
seasonal light minimum: (1) buds were formed and unfolded prem 
throughout the year, but the leaves were smaller and their daily growth less 
during the time of minimum light (Terminalia, Albizzia, Pithecolobium) ; (2) 
leaves were formed throughout the season, but these did not unfold normally 
from November to January (Theobroma); (3) plants rested during the months 
of minimum light (Eriodendron anfractuosum and probably Sterculia). 
The growth inhibiting or rest producing effect of diminished light may be 
shown in summer. This is most marked in complete darkness, which readily 
throws even A/lbizzia and Pithecolobium into rest. Albizsia continues to vege 
tate in darkness if it has much stored food, so that the effect of diminishing 
light seems to be related to reduced food supply and not to such specific action 
as Kress found for the beech. Limited light and limited salt supply may act 
together in producing rest. Albissia, Pithecolobium, and Terminalia were 
= Kress, G., Uber Wachstum und Ruhe tropischer Baumarten. Jahrb. Wiss. 
Bot. 56:734-792. 1915. 
