66 Wisconsin Academy of Sciences, Arts, and Letters. 
ly throughout the vegetative season the growth rings are not 
very marked though they are usually apparent. Generally the 
most reliable criterion for distinguishing the rings Is the reduc¬ 
tion in the radial diameter of at least the last row or two of 
wood cells; yet in the tropics histological distinctions are said 
to be practically absent in some trees, and their rings may only 
be distinguished by slight demarking lines. 
The work reviewed in this paper has shown that the environ¬ 
mental factors which control elongation growth also influence 
radial growth and that ordinarily the prevention of elongation 
by the removal of vegetative points hinders growth in thick¬ 
ness even when the environmental conditions are optimal and 
the food and water supply abundant. Klebs 130 assumed, in fact, 
that large quantities of organic foods accumulating in plants 
inactivates the enzymes concerned in elongation and therefore 
brings about a cessation of growth in length. According to 
him a timely increase in the water and inorganic nutrients may 
reactivate or prevent inactivation of the growth enzymes and 
thereby shorten or eliminate the dormant period. 
With such a precedent one may also assume the presence .of 
enzymes which incite and maintain radial growth since there 
are a number of phenomena to be noticed in connection with 
growth in thickness that support such an assumption, as may 
be gathered from the following papers. 
In an investigation on the reserve food in seeds Reiss 131 found 
that cellulose is laid down on the inner side of cell walls of 
many seeds and that it is largely redissolved on germination. 
Schulze 132 made a similar study of lupine seeds and found con¬ 
vincing evidence that the inner layers of the cotyledonary cell 
walls are used up during germination. It seemed that the dis¬ 
solving part of the walls is a hemiceliulose which gives rise to 
galactose and arabinose on hydrolysis. Griiss 133 also noted the 
occurrence of the hemicelluloses, galaetan and araban, in plant 
i33 G r (j SS) j. Ueber Losung und Bildung der aus Hemiceliulose besteh- 
enden Zellwande und ihre Beiziehung zur Gummosis. Biblio. Bot. 39. 
1896. pp. 14. 
130 1. c. 
131 Reiss, R. Ueber die Natur der Reservecellulose und liber ihre Auf- 
losungsweise bei der Keimung der Samen. Ber. Deut. Bot. Ges. 7: 322- 
29. 1889. 
132 Schulze, E. Ueber die Zellwandbestandtheile der Cotyledonen von 
Lupinus lutens und Lupinus angustifolius und liber ihr Verhalten wah- 
rend des Keimungsvorgangs. Ber. Deut. Bot. Ges. 14: 66-71. 1896. 
