554 SUMMARY AxND Chap. XU, 



pletely confluent, and they break through the ground 

 as an arch ; afterwards the petioles of the successively 

 formed early leaves are arched, and they are thus 

 enabled to break through the base of the confluent 

 petioles of the cotyledons. In the case of Megarrhiza, 

 it is the plumule which breaks as an arch through the 

 tube formed by the confluence of the cotyledon- 

 petioles. With mature plants, the flower-stems and 

 the leaves of some few species, and the rachis of 

 several ferns, as they emerge separately from the 

 ground, are likewise arched. 



The fact of so many difierent organs in plants of 

 many kinds breaking through the ground under the 

 form of an arch, shows that this must be in some 

 manner highly important to them. According to 

 Haberlandt, the tender growing apex' is thus saved 

 from abrasion, and this is probably the true explana- 

 tion. But as both legs of the arch grow, their power 

 of breaking through the ground will be much in- 

 creased as long as the tip remains within the seed- 

 coats and has a point of support. In the case of 

 monocotyledons the plumule or cotyledon is rarely 

 arched, as far as we have seen ; but this is the case 

 with the leaf-like cotyledon of the onion ; and the 

 crown of the arch is here strengthened by a special 

 protuberance. -In the Gramineae the summit of the 

 straight, sheath-like cotyledon is developed into a 

 hard sharp crest, which evidently serves for breaking 

 through the earth. With dicotyledons the arching of 

 the epicotyl or hypocotyl often appears as if it merely 

 resulted from the manner in which the parts are 

 packed within the seed; but it is doubtful whether 

 this is the whole of the truth in any case, and it cer- 

 tainly was not so in several cases, in which the arch- 

 ing was seen to commence after the parts had whollj 



