222 PASTORAL AND AGRICULTURAL BOTANY 



branches and may form very large clusters, e.g., Vicia faba and Stizo- 

 lobium, but there is one continuous bacteroidal zone, the apical portions 

 of which are traversed by innumerable infection threads. Two vascular 

 strands are produced at a very early stage of the development of the no 

 dule on opposite sides, each of which has a separate attachment to the root 

 stele. This group includes a number of plants of considerable agricul- 

 tural value viz., Vicia, Pisum, Lathyrus, Galega, Stizololium, and Colulea. 

 IV. The fourth group of nodules occur on plants such as Robinia, Sophora, 

 Acacia, of west temperate and subtropical regions. The nodules all 

 develop two vascular strands, which have a separate attachment to the 

 root bundle system and a well developed bundle sheath is present. In 

 Acacia, the nodule is bean-shaped, in Sophora and Robinia, the nodule 

 is transversely indented, the indentations occurring between two periods 

 of growth. This is the Mimosoideae type. 



The amount of nitrogen which is fixed by Bacillus radicicola has been 

 thought to be connected with the quantity of slime which is produced under 

 given conditions. If the formation of slime is great in amount the 

 bacteria are held in it and form a zoogleal thread. It is in this form, 

 that they enter the root hairs and passing from cell to cell finally reach 

 the root cortex. The slime is absorbed and the bacteria live freely in 

 the cell, being transformed into the so-called bacteroids, which are V and 

 Y shaped in such plants as Vicia faba, or spherical as- in Lotus corniculutus. 

 These it is believed are gradually absorbed by the plant. Recently, Erwin 

 F. Smith has called in question many of the accepted theories as to the 

 leguminous nodules, and he cites Gino-de-Rossi, who maintains that a 

 schizomycete of quite different character is the real cause of the nodules. 

 We have given the usually accepted views without presenting the con- 

 troversial points. 



Leaf Nodules of Rubiaceae. Recently attention has been called to 

 certain rubiaceous plants Psychotria bacteriophila and Pavetta Zimmerman- 

 niana and probably others, which have small nodules on their leaves, 

 which contain colonies of a non-motile, nitrogen-fixing bacterium named 

 by Faber Myco-bacterium rubiacearum. These bacteria almost invariably 

 inhabit the micropyle of the young seed, and, when the latter germinates, 

 grow through certain stomata of the young leaves and into the inter- 

 cellular spaces formed in the leaf-tissues around these stomata. Cavities 

 are formed through the growth of the epidermal cells which later close 

 entirely and make bacterial nodules which are deeply imbedded in the 



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