i 4 4 THE BOOK OF NATURE STUDY 



enough, be tied round with string or bast, so as to prevent 

 bolting, and to blanch the inner leaves. 



CRESS (Lepidium sativum) 



This is best grown in boxes, and is therefore equally as suitable 

 for cultivation in town as in country schools. Any small wooden 

 box may be used. The box should be filled with a mixture of 

 good garden soil and leaf-mould pressed firmly down, and so 

 that its surface is about one inch from the top of the box. The 

 seed is sown thickly on the surface, a little soil is sprinkled thinly 

 over them and gently pressed down. The box is then covered 

 over with paper until the seedlings begin to appear. The Cress 

 will be ready for cutting in ten days or a fortnight from the time 

 of sowing. In the early part of the spring the Cress boxes should 

 be kept in the schoolroom. Later they may stand out of doors. 



BEANS AND PEAS 



These plants belong to the natural order Leguminosae and the 

 sub-order Papilionaceae. They all possess the power of assimilat- 

 ing free nitrogen from the mixture of gases present in the soil 

 spaces. This they do through the agency of the Bacterium 

 radicicola with which they have a symbiotic relationship. If 

 one of these plants is carefully dug up by the roots and well 

 washed until free from soil particles there will be seen on the 

 roots numerous small round pink or white bodies varying in size 

 from a pin's head to a small pea. These are the nodules. A 

 cross section of a nodule when mounted or viewed under the 

 microscope shows a central mass of parenchymatous cells packed 

 with rod-shaped and Y-shaped bacteria. When the plant gets 

 old the nodules become disintegrated, and the bacteria become free 

 in the soil. When new plants are grown in the soil the bacteria 

 make their way through the root epidermis and again give rise 

 to the nodular excrescences. Thus in a soil rich in this species 

 of bacterium, leguminous plants are independent of the nitrates 

 present in the soil; and further, owing to the accumulation of 

 nitrogenous compounds in the roots, such plants may leave a 



