148 PEACTICAL BOTANY 



containing a new plant. Defined from its function, it is a highly 

 portable and not easily injured package, in which the rudi- 

 ments of a plant, like its parent, may be carried about and hold 

 life over from season to season. 



136. Need of seed distribution. The successive crops of farm 

 and garden annuals are secured by careful seed planting in 

 prepared soil. The seeds of wild plants are also sown, on a 

 still more extensive scale, by natural agencies. In any country 

 the relative numbers of most kinds of wild seed plants usually 

 remain from year to year without great changes except those 

 which are brought about by human interference. This fact is 

 evidence enough that seeds in unimaginable numbers must 

 be scattered about in such a way as to make good the losses 

 in the plant population of the world due to all destructive 

 causes. The means of seed distribution will be taken up in 

 Sects. 140 and 141. 



137. The struggle for existence. Only a small proportion of 

 all the seeds annually produced can have a chance to grow. 

 The resulting contest among plants for a foothold and for 

 the means of subsistence forms one portion of what the great 

 English naturalist, Charles Darwin, called the struggle for exist- 

 ence. It is shown by careful calculation that about 5,300,000 

 acres of land could be sown with the wheat grown at the end 

 of fifteen years from a single parent kernel, if every grain 

 were to grow and live. But the wheat plant does not produce 

 a very large number of seeds. The so-called Russian thistle 

 {Salsola Kali, var. tenuifolia), a most troublesome weed, bears 

 from 20,000 to 200,000 seeds. Taking the moderate estimate 

 of 25,000 seeds to a plant, their offspring (if all the seeds grew) 

 would number 625,000,000 individuals, and the next genera- 

 tion would number 15,625,000,000,000. Supposing each plant 

 to have a diameter of about three feet and to occupy an area 

 of seven square feet, the student can readily calculate how 

 many square miles of territory the number of plants last 

 named would cover, if actually in contact with each other 

 throughout their circumferences. 



