. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. MORPHOLOGY. 23. Involution Forms. Under this name we designate swollen and distorted forms comnion in old cultures (fig. 21). Under wliat conditions do they occur ? Are the}- living or dead ? Isolate in hanging drops of bouillon and detennine whether they are stages in development or only degenera- tion forms. Are Y-shaped or branched fonns such as occur in old cultures of B. tuberculosis Koch, and in the root-tubercles of clover (fig. 22) to be considered as involution forms ? Are such organisms fungi or bacteria ? Branch
. Bacteria in relation to plant diseases. Bacteriology; Plant diseases. MORPHOLOGY. 23. Involution Forms. Under this name we designate swollen and distorted forms comnion in old cultures (fig. 21). Under wliat conditions do they occur ? Are the}- living or dead ? Isolate in hanging drops of bouillon and detennine whether they are stages in development or only degenera- tion forms. Are Y-shaped or branched fonns such as occur in old cultures of B. tuberculosis Koch, and in the root-tubercles of clover (fig. 22) to be considered as involution forms ? Are such organisms fungi or bacteria ? Branching forms have been detected by many obsen-ers. (Consult numerous citations in the Bibliography of General Literature, X). The most recent paper is b)- Albert Fig- 18* Maassen (Arb. a. d. Kais. Gesundh., Bd. XXI, H. 3, 1904, p. 377, 6 pi.). He found chloride of lithium specially advantageous for provoking these growths, which are re- garded as teratological. He obtained them in 24 hours. GENERAL COMMENT. Great care should be paid to the minute morphology of each organism, not only in the host-plant but also in a variety of cultures, old and voung, so that a bod\- of knowledge more exact than we now possess shall be grad- ually accumulated for differential and systematic purposes. Careful drawings and photographs should be made. The Abbe camera is a great help in making drawings (fig. 121). For such study the Zeiss apochromatic lenses and com- pensating oculars can not be recommended too highly, particularly the 16 mm., with the 12 and 18 compensating oculars for studying the margins of colonies, and the 2 mm. n. ap., with the 8 and 12 compeusating oculars for the more detailed study of the indi\-idual rods. The writer has also made much use of the Zeiss 3 mm. n. ap. apochro- matic objective. The Zeiss screw, or filar, micrometer com- bined with a No. 12 compensating ocular (fig. 23) will be found \-er}- useful. For photographic purposes the projec- tion oculars or the
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