DEPARTMENT OF BOTANICAL RESEARCH. 59 



Light of the shorter wave-lengths (blue-violet rays) has a further 

 possible effect on growth in that it coagulates or neutralizes colloids 

 (suspensoids) and reduces their hydratation capacity, and may also 

 possibly break down some nuclear substances. The radiation in 

 question, however, has the least penetrating power of any part of the 

 spectrum, and hence its action is most marked in minute organisms, or 

 those with translucent membranes. Marked morphogenic alterations 

 in the higher plants may be induced by illumination from a mercury- 

 vapor arc in a quartz tube. 



Methods and Material for Study of Fundamental Processes of Growth, 



by D. T. MacDougal. 



Nearly all available data concerning growth of the higher plants 

 have been obtained by a study of single organs of seedlings, measure- 

 ments being made under conditions not capable of jdelding conclusions 

 of wide application, or which afford a basis of analj^sis of the contribu- 

 tory processes. 



An inspection of the material available at the Desert Laboratory 

 made it apparent that the flattened opuntias offered certain structural 

 features and habits of growth which promised unexcelled opportuni- 

 ties for analyses of some of the physico-chemical processes of growth 

 and to make exact measurements subject to well-defined corrections. 

 The joints of these plants emerge as small flattened buds, thickly 

 covered with ephemeral leaves, and they expand to a length of 15 to 

 20 cm. and a width of 8 to 16 cm. in about two months. 



The growing joints are firm enough to give steady contacts with 

 counter-weighted levers carrying a pen which traces the expansion or 

 other change in volume continuously. The joints are complex as to 

 morphogeny, but enlarge as simple but heterogeneous disks of plas- 

 matic colloid against distensible membranes and fibro-vascular skele- 

 tons. The transpiration, stomatal action, respiration, and the major 

 features of sugar content and acidity of these plants are known, so that 

 it was possible to connect alterations in rate of growth with chemical 

 conditions in the joint, a connection not hitherto adequately established. 

 Instead of a single series of chambers with temperature control, it has 

 been found most effective to establish separate small chambers at the 

 various small experimental plants. Light screens of various trans- 

 missibility as described in the previous section have also been devised 

 and constructed. The compound-lever systems used to obtain the 

 auxographs of some of these plants were adjusted to record all varia- 

 tions in the dimension being measured. The slackened elongation 

 rate and the shrinkages detected in the tracings, which in some cases 

 extended for ten v/eeks without break, furnished evidence upon which 

 important contributions may be made as to the relation of acidity and 

 respiration to growth. 



