114 PLANT BIOLOGY 



ready penetration of the roots of the growing plant is possible. 

 In small garden plots this work is done by the use of spades, 

 hoes, and rakes. It is often found necessary to add well- 

 rotted manures to increase the humus of the soil and chemi- 

 cally prepared fertilizers, which furnish available mineral 



food for the crops. We 

 have already called at- 

 tention to the necessity 

 of proper drainage of 

 the soil before crops are 

 planted (122). Scien- 

 tific investigation has 

 demonstrated, too, that 

 frequent and thorough 



stirring of the soil is most important not only to prevent 

 the growth of weeds, but also, and this is even more 

 essential, to conserve the soil moisture, and insure proper 

 aeration of the roots. It has been found that it is possible 

 to produce large crops on semiarid land if the top-surface of 

 the ground is kept in a thoroughly pulverized condition. 

 This is the so-called method of " dry farming." 



L 



IV. THE STRUGGLE FOR EXISTENCE AND ITS EFFECTS 



126. Variation among plants. We have all heard the common 

 expression "as nearly alike as two peas." In reality, however, 

 if our powers of observation were sharp enough, we should probably 

 find that no two peas are exactly alike in shape, color, size, and 

 weight. The plants grown side by side from any two peas would 

 also vary in height, in number and position of leaves, and in the 

 number and vigor of flowers and seeds. In other words, as every 

 human being has certain distinguishing characteristics, so, too, we 

 should bear in mind that every individual plant, however small, 

 shows certain differences or variations from every other individual 

 of its class. 



