334 HOW CEOPS GROW. 



These leaves are folded together along their -mid-ribs, and 

 may be opened and spread out with help of a needle. 



When the kidney-bean (Phaseolus) germinates, the 

 cotyledons are carried up into the air, where they become 

 green and constitute the first pair of leaves of the new 

 plant. The second pair are the tiny leaves of the plum- 

 ule just described, between which is the bud, whence all 

 the subsequent aerial organs develop in succession. 



In the horse-bean ( Vicia faba), as in the pea, the cot- 

 yledons never assume the office of leaves, but remain in 

 the soil and gradually yield a large share of their con- 

 tents to the growing plant, shriveling and shrinking 

 greatly in' bulk, and finally falling away and passing into 

 decay. 



§3. 



VITALITY OF SEEDS AND THEIR INFLITBKCE ON THE 

 PLANTS THBX PEODXJCE. 



Duration of Vitality. — In the mature seed the em- 

 bryo lies dormant. The duration of its vitality is very 

 various. The seeds of the willow, it is asserted, will not 

 grow after having once become dry, but must be sown ' 

 when fresh ; they lose their germinative power in two 

 weeks after ripening. 



On the other hand, single seeds of various plants, as of 

 sorrel {Oxalis stricta), shepherd's purse {Thlaspi arv- 

 ense), and especially of trees like the oak, beech, and 

 cherry, remain with moist embryos many months or sev- 

 eral years before sprouting. (Nobbe & Haenlein, 7s. 

 St., XX, p. 79.) 



Among the seeds of various plants, clover for example, 

 which, under favorable circumstances, mostly germinate 

 within one or two weeks, may often be found a number 

 which remain unchanged, sound and dry within, for 

 months or years, though constantly wet externally. The 



