SUMMARY: A simple glass slide technique has been devised for the continuous microscopical observation of growth and infection of root hairs of clover seedlings. The method involves an aseptic cultivation of seedlings on microscope slides which are partly immersed in a mineral salts medium. The roots are protected by a coverslip.By this procedure, the root hairs of white clover inoculated with nodule bacteria were studied. The earliest infection was observed to take place within 48 hr. of inoculation, on 4-day-old seedlings. In branched hairs the growth of the thread from a lateral branch towards the hair tip is tentatively explained as an effect of the position of the hair nucleus relative to the site of infection.The infection of leguminous plants by nodule bacteria has, as a rule, been studied with fixed and sectioned material. The first stages of this process may also be studied in vivo, but no suitable technique has yet been described for a continuous microscopical examination of these stages.A method for the study of growing root hairs (of wheat) was devised by Lundegkrdh (1946) and modified by Ekdahl(1953). In their work, a chamber containing flowing salt solution and resting on a microscope stage was employed. However, difficulties arise with this method if aseptic conditions are also necessary. The present author has therefore used a technique for the aseptic cultivation of young seedlings on microscope slides in a manner which permits periodic observations under the microscope of the growth and infection of individual root hairs. The arrangement also offers good conditions for photomicrography .The details of the method are as follows. METHODA nitrogen-free mineral solution of the following composition was prepared :CaCl,, 0.1 g.; MgSO4.7H,O, 0.12 g.; KH,PO,, 0.1 g.; Na,HPO,.ZH,O, 0.15 g.;Fe citrate, 0.005 g.; Mn, Cu, Zn, B, Mo traces; dist. water, 1000 ml.; pH 6.5 (after autoclaving).Portions (25 ml.) of this solution were distributed into large glass tubes (39 x 125 mm.). The tubes were plugged with cotton wool or covered with closely fitting glass caps (45 x 60 mm.) and were sterilized in the autoclave for 20 min. at 120" (15 lb./sq.in.). Microscope slides (26 x 75 mm.) and cover-slips (24 x 40 mm.) were sterilized by heating dry in Petri dishes, preferably only one slide and one cover-slip in each dish.

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SUMMARYThe production of pol ygalacturonase in associations of nodule bacteria and seedlings of leguminous plants was investigated. Plants and bacteria were combined in the following ways : ( a ) different plant species were combined with infective strains (isolated from the same cross inoculation group) as well as non-infective ones (from foreign groups) ; ( b ) host plant species with different susceptibilities were combined with the same bacterial strain; ( c ) bacterial strains with different infectivities, as measured by the number of infection sites, were combined with the same host species; ( d ) clover strains which had lost their infectivity, as well as transformed and again infective subcultures of these strains, were tested on clover plants. The results indicate that infection of the seedlings was strongly correlated with the production of polygalacturonase. The conclusion is drawn that polygalacturonase plays an important part in the infection process. This function is thought to be a weakening of the cell wall of the root hair which would facilitate the bacterial invasion. The possible role also of indolyl-$-acetic acid in the infection process is discussed.

SUMMARYRoot hairs of clover seedlings infected by Rhizobium were studied in the electron microscope. The appearance of the infection sites supports the invagination hypothesis proposed by Nutman. The origin of the material enclosing the bacteria within the infection thread is discussed.

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