THK DEPTH IN THE SOIL AT WHICH PLANTS OCCUR. 187 



the process the representative of the future plant (i.e. the 

 plumule and radicle) may be carried some little distance down, 

 at any rate a few millimetres into the ground. Not too far, 

 however, for with the limited resources at its disposal grave diffi- 

 culties might arise in bringing the first leaves up to the light. 

 This is often the case when the cotyledon or cotyledons 

 are hypogeal, and is particularly notable amongst the Mono- 

 cotyledons, in which the apical part of the cotyledon generally 

 remains as a sucker within the seed, whilst its basal sheathing 

 portion emerges, and stretching pushes the radicle and plumule 

 some little distance into the soil. From this position the first 

 leaves are unfolded, and various means are employed in different 

 cases to bring the young plant to a deeper level. It is with 

 these methods that the present paper is concerned. 



1. RoOT-SHOliTHNING. 



Not the sole, but by far the most important agent in this 

 process is the contractility of the root. As this particular 

 property of roots is a little overlooked in the usual sources of 

 information, it will be convenient to indicate the main features 

 of the phenomenon. 



A shortening or contraction of the root on the attainment of 

 a certain age is of frequent occurrence, and in well-marked cases 

 is associated with a transverse wrinkling, which is quite unmis- 

 takable. This commences in the oldest part of a root, some 

 time after growth in length has ceased in the region concerned, 

 and spreads in the direction of the apex. This is well shown in 

 fig. 113, representing the root of Carum Carvi, as also in fig. 114, 

 representing successive stages in the life-history of the Amaryl- 

 lidaceous Phcedranassa chloracea. The general phenomenon of 

 root-shortening has been the subject of investigations by 

 de Vries,* Stroever,t and Rimbach,t and the main facts in the 

 operation may be briefly summarised as follows : — 



Many roots exhibit in those parts which have ceased to grow 

 in length a definite and measurable contraction, owing to a 

 remarkable change in form in the cortical cells. These still 



* De Vries, Landw. Jahrb. 1880. 



f V. Stroever, Ueb. d. Verbreitung d. Wurzelverkiirzung, Inaug. Diss. 

 Jena, 1892. 



\ A. Rimbach, numerous papers in Bar. d. deutsch. bot. Gcs. vols. xi. 

 to xv.; also Fiinfstiick's Beitr. zur Wissensch. Bot. Band li. p. 1. 



