92 



KNOWLEDGE. 



[April 2, 1894. 



lATeR^L ROOTLET 



Fig. 3. — Cross section 

 of Tubercle on root 

 of Bean. 



origin from the root the cells are filled with small cor- 

 puscles, which have been called bacteroids, from their 

 resemblance to bacteria. If the specimen under examina- 

 tion is fresh, these corpuscles are to be seen in active 

 motion, but a high power of the microscope is necessary 

 to observe them properly. The corpuscles are of various 

 forms, as shown in Fig. 4 ; some are spherical, others are 

 larger and shaped like the letter Y- The spherical cor- 

 puscles were found in tubercles formed some time ago ; 

 the Y-shaped in the freshly-formed 

 nodules. A much magnified view of 

 some cells filled with bacteroids is 

 given in Fig. 5. They were taken 

 horn, the inner portion of the tubercle 

 shown in Fig. 3. The densely granu- 

 lar nature of the cell contents is due 

 to the enormous development of the 

 bacteroids. These have preyed upon 

 the cell contents, causing a flow of 

 nourishing matter to the cells, re- 

 sulting in their abnormal enlarge- 

 ment. The open spaces in the cells 

 are termed "vacuoles"; they are filled with a watery 

 fluid. The black body is spoken of as the " nucleus." 



Tubercles have been found on the roots of all leguminous 

 plants when grown in the open ; but under certain experi- 

 mental conditions the tubercles are not to be found. Thus 

 when the seeds are germinated in sand or cocoanut fibre 

 (sterilized), and are then carefully washed and watered 

 with distilled water, containing nourishing salts in solution, 

 the young plants will grow, if well provided with all the 

 elements necessary for their develop- 

 ment. But no tubercles will be 



formed when these precautions are jvj^ _ ' o_^ C. 



carefully observed, and the water 

 used has been properly sterilized. If, 

 however, the plant is infected by 

 contact with other tubercles, or a 

 small quantity of garden soil be in- 

 troduced into the culture medium, Tj; . 

 the formation of nodules at once 

 takes place, and it has been 

 shown by Prof. Marshall Ward that 

 the act of infection is a perfectly definite one. Figs. 6, 

 7, and 8 show that the infection consists in the 

 distortion of the cell-wall of a root-hair at one portion 

 where a brilliant dot is noticed. From this point a 

 fine tube passes down the cavity of the root-hair. It 

 then passes through the cell-wall and into the underlying 



cortical cells, its tip ulti- 

 mately reaching the inner- 

 most cells of this portion of 

 the root. There it branches in- 

 to the neighbouring cells , and 

 the irritation set up causes 

 them to renew their activity ; 

 tliey divide up and form the 

 mass of the tubercle. In the 

 cell- wall of one of the cells 

 depicted in Fig. 6 the hypha 

 is first seen to branch, and 

 it is not an uncommon thing 

 to find, as in one of these 

 branches, the hypha attached to the nucleus. Where the 

 hyphae penetrate the walls of the cells peculiar trumpet- 

 shaped widenings may be noticed, suggesting the idea that 

 after the hyphfe had passed through them the cell-walls of 

 the still growing cells had stretched out. Eeferring to 

 Fig. 8, we see a peculiar " trumpet- shaped widening" of 



V 



> 



^'- 



b. 



Fig. 4. — Bacteroids from 

 cells of Tubercles. 



Fig. 5. — Much magnified section 

 of cells containing Bacteroids. 



the cell-wall of the root-hair where the organism has 



obtained an entrance into the interior. 



B So far, then, we have seen that peculiar brilliant spots 



are noticeable on certain of the root-hairs of plants affected 



by this organism ; 

 that from these a 

 tube passes down into 

 the tissues of the 

 root ; that it branches 

 in the inner layers of 

 the cortex, and stimu- 

 lates the attached 

 cells much in the 

 same way as does 

 PlasuHiiJiopliorn hras- 

 sic(B, the cause of the 

 disease known as 

 club-root, or fingers 

 and toes on the turnip 

 and allied plants. 

 We have also seen 

 that in the outer cells 

 of a still healthy 

 tubercle hyphffi are 

 present along with 

 minute corpuscular 

 bodies, which are 

 termed "bacteroids," 

 and that, further, the 

 bacteroids found in a 

 tubercle of the current 

 year are peculiarly 

 shaped, whilst those 

 fomid in old tubercles 

 are spherical. 



Fig. 6. — Section of root showing 

 hypha penetrating cells. 



We now come to the examination of a preparation which 

 helps to throw some light on the formation of the 

 bacteroids. If we turn to Fig. 9 we shall see a very much 

 magnified view of a few cells from the dark shaded portion 

 of the tubercle of Fig. 3. The 

 protoplasm of the cells is very 

 granular with bacteroids, and in the 

 majority of cases contains many 

 vacuoles. Furthermore, two branch- 

 ing hyphfE are visible, which appear 

 to be budding off from their tips 

 minute bacteroids. At this stage it 

 appears that starch often accumu- 

 lates in the cells of the tubercle, 

 and that the nucleus occasionally 

 undergoes a fatty degeneration. 

 The widening of the hypha in its 

 passage through the cell-wall is 

 more clearly seen in the next figure 

 (Fig. 10), and in the subsequent 

 ones. Figs. 11 and 12, we have a 

 more enlarged view of the process 

 of budding off of the bacteroids at 

 the tips of the hyphae. 



In the Botanischi's Cent ralblatt for 

 1888 there is a paper of Prof. 

 Prazmowski's which confirms in the 

 main the statements of the life- 

 history as worked out by Prof. 

 Marshall Ward, from whose paper 

 in the PhilosopJiical Transactions 



we have borrowed our illustrations, -c ., a i- c 



, . , ,.„ . J i. -1 Fig 7. — Section of 



but he differs in some details, root.hair and cells, ivith 



Marshall Ward, for example, states p»netrating hypha. 



