BACTERIA AND DISEASES OF ANIMALS 803 



application to the seed and soil, and the influence of dryness 

 and sunlight upon them, before the organisms of 'nitragin' 

 cultures can be successfully utilised in farm practice. 



Ex. 329. — Carefully dig up various leguminous plants just before flowering 

 and examine the colour, form, and number of the nodules on the roots of 

 each. 



Ex. 330. — Examine the nodules on the roots of the bean, pea, or vetch at 

 different short intervals from the time they first appear on the seedling to the 

 time when the plant bears perfectly ripe seeds. Note the difference in size 

 and the external and internal colour of the nodules at each stage. 



Ex. 331. — Cut transverse sections of plump nodules of the bean and note 

 the position of the various tissues with a low power {cf. A, Fig. 265). 



Ex. 332. — Cut across a well-developed nodule of a bean and scrape off 

 a minute portion of the exposed bacteroidal tissue with the point of a knife ; 

 place the scraped portion in a very small drop of water on a glass slide and 

 stir it about, so that the bacteroids are distributed in the water ; put on a 

 cover-slip and examine with a one-eighth objective. 



Ex. 333. — Make three sand-cultures of peas, adding all necessary plant 

 food-constituents except nitrogen ; water one of them {A) always with distilled 

 water ; another (5), first with a thoroughly boiled extract of garden soil 

 on which peas have been grown, and subsequently with distilled water ; and 

 the third (C), first with a similar extract of garden soil which has not been 

 boiled, and afterwards with distilled water. 



Watch and make notes on the growth of each, and when six or seven 

 weeks' old, take out the roots of all the plants from the sand and observe 

 which bear nodules. 



Ex. 334. — Make water-cultures of peas or beans, similar to the [A) and (C) 

 sand-cultures of Ex. 333. Instead of adding extract of soil to (C), bruise a 

 number of well-developed pea or bean nodules, and add them to the water in 

 the experimental jar when several lateral roots of the plants are half an inch 

 or more long ; also place some of the bruised nodules in contact with the 

 young roots. 



Watch and make notes on the subsequent growth of the plants, and the 

 presence or absence of nodules on each. 



12. Bacteria and diseases of animals. — The bacteria pre- 

 viously discussed have been those which live a saprophytic 

 life, deriving their food from dead organic matter, and at the 

 same time bringing about extensive chemical changes in it. A 

 number of species, however, gain an entrance into the bodies of 

 animals where they feed upon the substances of the tissues, and 



