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; vet in the tropics histological distinctions are said 

 to he practically absent in some trees, and their rings may only 

 !>< distinguished by slight demarking lines. 



The work reviewed in this paper has shown that the environ- 

 i-ii-ntal factors which control elongation growth also influence 

 radial growth and that ordinarily the prevention of elongation 

 by tlie removal of vegetative points hinders growth in thiek- 

 hess even when the environmental conditions are optimal and 

 tin 1 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 

 react i\;>te or prevent inactivatioii 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 hemicellulose which gis-es rise to 

 galactese and arabinose on hydrolysis. Griiss 133 also noted the 

 occurrence of the hemicelluloses, galactan and araban, in plant 



33 Griiss, J. Ueber Losung und Bildung der aus Hemicellulose besteh- 

 enden Zellwande und ihre Beiziehung zur Gurnmosis. Biblio. Bot. 39. 

 1896. pp. 14. 



110 1. c. 



131 Reiss, R. Ueber die Natur der Reservecellulose imd iiber ihre Auf- 

 losungsTveise 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 iiber ihr Verhalten wah- 

 rend des Keimungsvorgangs. Ber. Deut. Bot. Ges. 14: 66-71. 1896. 



