Chap, XII. CONCLUDING EEMAKKS. 555 



escaped from the seed-coats. As the arching occui-red 

 in whatever position the seeds were placed, it is no 

 doubt due to temporarily increased growth of the 

 nature of epinasty or hyponasty along one side of the 

 part. 



As this habit of the hypocotyl to arch itself appears 

 to be universal, it is probably of very ancient origin. 

 It is therefore not surprising that it should be in- 

 herited, at least to some extent, by plants having 

 hypogean cotyledons, in which the hypocotyl is only 

 slightly developed and never protrudes above the 

 ground, and in which the arching is of course now 

 quite useless. This tendency explains, as we have 

 seen, the curvature of the hypocotyl (and the conse- 

 quent movement of the radicle) which was first 

 observed by Sachs, and which we have often had to 

 refer to as Sachs' curvature. 



The several foregoing arched organs are continually 

 circumnutating, or endeavouring to circumnutate, even 

 before they break through the ground. As soon as 

 any part of the arch protrudes from the seed-coats it 

 is acted upon by apogeotropism, and both the legs 

 bend upwards as quickly as the surrounding earth will 

 permit, until the arch stands vertically. By continued 

 growth it then forcibly breaks through the ground; 

 but as it is continually striving to circumnutate this 

 will aid its emergence in some slight degree, for we 

 know that a circumnutating hypocotyl can push away 

 damp sand on all sides. As soon as the faintest ray of 

 light reaches a seedling, heliotropism will guide it 

 through any crack in the soil, or through an entangled 

 mass of overlying vegetation; for apogeotropism by 

 itself can direct the seedling only blindly upwards. 

 Hence probably it is that sensitiveness to light resides 

 in the tip of the cotyledons of the Gramineas, and in 



