THE MOLECULAR FORCES IN PLANTS. 193 



cells of which are tangentially elongated, and lastly the remains 

 of the compressed endosperm. 



The soaking of most seeds is effected by imbibition and 

 osmosis. Tn some cases, e.g. the seeds of Linum usitatissimum, 

 the process is accelerated by the formation in contact with water 

 of a mucilage, which very energetically attracts and retains the 

 water. In fact, each Linum seed at once surrounds itself, when 

 brought into contact with water, with a sheath of mucilage. We 

 prepare very delicate transverse sections of dry Linum seeds, and 

 mount them in alcohol. Now, observing the result, we run in 

 water from the edge of the cover-glass. At the moment when 

 the water reaches the section, the epidermal cells of the seed- 

 coat swell up very strongly, and exhibit the mucilage present as 

 thickening layers in their outer walls, the cell-walls remaining 

 intact. These epidermal cells, as we now easily see, are arranged 

 perpendicularly to the outer surface of the seed. We will not 

 here consider further the complicated structure of the seed-coat 

 of Linum. 



If we examine a dry Phaseolus seed, and place it so that the 

 hilum, which appears as a white strip, is directed towards the 

 observer, we find at one side of the strip a small hole, the 

 micropyle, which lies immediately over the tip of the root of the 

 embryo. At the opposite end of the hilum we see two small 

 swellings (Doppeltuberkeln), separated by a shallow commissure. 

 These, with the hilum and the micropyle, form the hilary apparatus 

 of the seed. The micropyle plays a particularly important role in 

 the absorption of water by bean- seeds, as the following experi- 

 ment shows. One Phaseolus seed, a, is completely immersed in 

 water. A second seed, 6, as nearly as possible equal to a in 

 weight, is fixed in a suitable manner on a needle, and placed in 

 water in such a way that the hilary apparatus is not wetted. If 

 we weigh after a few hours, it will be found that a has taken 

 up a comparatively large quantity of water, while 6 has ab- 

 sorbed but little. 2 



1 See Detraer, Journal f. Landivirthschaft, 27. Jahrgang, p. 119. 



2 On the structure of seed-coats, see especially Sempolowski's Dissertation, 

 Leipsic, 1874. There also the most important literature is given. See also 

 Mattirola and Buscalioni, Memorie della E. Accadcmia delle Scienze di Torino, 

 Serie II., T. xlii. 



P.P. O 



