REPRODUCTION. 235 



especially birds, may carry these seeds great distances on land or 

 across the water. 



In some cases the arillus has an entirely different function. 

 Among Leguminosce it forms a scission tissue between the placenta 

 and the seed, causing delicate tissues to rupture. In Nymphcea 

 the arillus serves to keep the seed afloat. The seed floats upon 

 the surface of the water for about forty-eight hours, so long as 

 there is air in the cavity of the arillus ; as soon as water displaces 

 the air the seed takes a position with its apex upward and falls out 

 of the sac to the bottom of the water, where it begins to germinate. 

 In various families (Berber idacem, Turneracece) the arillus serves 

 the same function as the winged appendages of seeds. 1 



Leaving out of consideration the arillus, which is not always 

 present, we have yet to discuss the seed-coat.' Sometimes we may 

 distinguish two layers, an inner (tegmen) and an outer (testa), 

 which, however do not always originate as two separate coats 

 (RADLKOFER). The above-mentioned winged and hair-like ap- 

 pendages are products of the seed-coats. In some seeds there are 

 still other hair-like appendages which serve to attach the seed to 

 the soil during germination. 3 This is also the case in some fruits. 

 The mucilaginous cell- walls of the outer seed-coat serve a similar 

 purpose, as in Linum usisatissimum, OrucifercB, Labiatce. The 

 mucilage also retards the evaporation of moisture from the seed 

 (KLEBS). 



The endosperm, which we have already learned to know, needs 

 to be considered more from a physiological standpoint, especially in 

 connection with the discussion of seed- and fruit-coats. Commu- 

 nications and citations of literature in regard to this subject are 

 found with MARLOTH.* In 1890 "W. HIRSCH published a commu- 



1 PLANCHON, BAILLON, HILDEBRAND, BACHMANN, PFEIFFER, and others, 

 from whom we have taken the foregoing statements, made special studies of thi& 

 subject. 



2 See PPEFFER'S Untersuchungen a. d. Botanischen Institut zu Tubingen. 

 FRANK gives numerous citations to the literature of this subject in his Lehrbuch 

 der Botanik, p. 159 (1892). See also R. LOOSE, Die Bedeutung der Frucht und 

 Sameuschale, etc., Berlin, 1891. 



3 GRUTTER, W., liber den Bau und die Entwickelung der Samenschalen eiui- 

 ger Lythrarieen, Bot. Zeitung, 1893. TRANS. 



4 Uber mechanische Schutzmittel der Samen, etc., Botanische Jahrbucher IV> 

 1883. 



