Chap. XII CONCLUDING KEMAEKS. 55S 



arrangement of the three orders of roots is ex3ellently 

 adapted for searching the whole soil for nutriment. 



Sachs has shown that if the tip of the primary- 

 radicle is cut off (and the tip will occasionally be 

 gnawed off with seedlings in a state of nature) one of 

 the secondary radicles grows perpendicularly down- 

 wards, in a manner which is analogous to the upward 

 growth of a lateral shoot after the amputation of 

 the leading shoot. We have seen with radicles of the 

 bean that if the primary radicle is merely compressed 

 instead of being cut off, so that an excess of sap is 

 directed into the secondary radicles, their natural con- 

 dition is disturbed and they grow downwards. Other 

 analogous facts have been given. As anything which 

 disturbs the constitution is apt to lead to reversion, 

 that is, to the resumption of a former character, it 

 appears probable that when secondary radicles grow 

 downwards or lateral shoots upwards, they revert to 

 the primary manner of growth proper to radicles and 

 shoots. 



With dicotyledonous seeds, after the protrusion of 

 the radicle, the hypocotyl breaks through the seed- 

 coats; but if the cotyledons are hypogean, it is the 

 epicotyl which breaks forth. These organs are at first 

 invariably arched, with the upper part bent back 

 parallel to the lower ; and they retain this form until 

 they have risen above the ground. In some cases, 

 however, it is the petioles of the cotyledons or of the 

 first true leaves which break through the seed-coats 

 as well as the ground, before any part of the stem 

 protrudes ; and then the petioles are almost invariabl v 

 arched. We have met with only one exception, and that 

 only a partial one, namely, with the petioles of the two 

 first leaves of Acanthus candelabrum. With Delphinium 

 nudicaule the petioles of the two cotyledons are com- 



