REPRODUCTION. 235 



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

 across the water. 



In some cases the arilhis has an entirely different function. 

 Among Legmninosce it forms a scission tissue between the placenta, 

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

 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 {Berhericlacece^ Taymeracece) the arillus serves 

 the same function as the winged appendages of seeds.' 



Leaving out of consideration the arillus, which is not always 

 present, we have yet to discuss tlie 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 

 (Radlkofek). 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.^ Tliis is also the case in some fruits. 

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

 purpose, as in Linum usisatissimum.^ Cruciferoe., Lahiatce. The 

 mucilage also retards the evaporation of moisture from the seed 

 (Klebs). 



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

 to be considered more from a physiological standpoint, especiall}^ in 

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

 nications and citations of literature in regard to this subject are 

 found with Makloth.* In 1890 W. Hiesch published a connnu- 



' Planchon, Baillon, Hildebrand, Bachmann, Pfeiffer, and others, 

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

 subject. 



- See Ppeffer's Untersuchungeu a. d. Botanischen Institut zu Tubingen 

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

 der Botanik, p. 159 (1893). See also R. Loose, Die Bedeutung der Frucht uud 

 Samenschale, etc., Berlin, 1891. 



* GR0TTER, W., ijber den Bau und die Entwickelung der Samenschalen eini- 

 ger Lythrarieen, Bot. Zeitung, 1893. — Trans. 



■• iJber mechanische Schutzmittel der Sanien, etc., Botanische Jahrbiicher IV, 

 1883. 



