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A TEXTBOOK OF THEORETICAL BOTANY 



but many seeds have long hairs 

 attached to one end of the seed 

 {e.g., Epilohiiim, Salix, AscJepias) 

 which aid in the dispersal of the 

 seeds by wind. These outgrowths 

 may be classed as arils. They arise 

 sometimes around the micropyle 

 {Asclepias) or from the chalaza 

 {Epilobiiiin). Exceptionally the testa 

 may be soft, thick and fleshy or 

 gelatinous, instead of hard. A good 

 example of the former is the seed 

 of Magnolia, and of the latter that 

 of Piinica granatiim, the Pome- 

 granate. Such seeds are usually 

 dispersed by birds. Mucilage coat- 

 ings will be dealt with later. 



Fig. 1357. — Vanilla planifolia. Longi- 

 tudinal section of mature seed. The 

 outer integument remains intact, 

 its outer layers becoming sclerotic 

 and opaque. The inner integument 

 is only a thin, crushed layer round 

 the embryo. Note the prominent 

 hypostase at the chalazal end. 

 {After Sivamy.) 



'M 



• • 



Fig. 1356. — Odontoglossiiin sp. Ripe seeds 

 with loose, membranous testas and minute 

 undifferentiated embryos. 



The seeds of Orchids (Fig. 1356) 

 (with the exception of those genera 

 having fleshy fruits, such as Vanilla 

 (Fig. 1357)) are exceptional in having 

 a thin, papery and transparent testa, 

 consisting of the epidermis of the outer 

 integument, which forms a loose sack. 

 The outer cell walls usually disappear, 

 but the side and inner walls are cuti- 

 cularized. The protection of the em- 

 bryo is assisted by the development 

 of a cuticle around the embryo itself. 



Thin and papery testas are also 

 characteristic of the solitary seeds of 

 drupe fruits and other indehiscent 

 fruits such as akenes, in which the 

 protective function has been assumed 

 by the stony endocarps or the pericarp 

 as a whole. Likewise, in most Gram- 

 ineae and Compositae the integuments 

 do not develop into a testa but dis- 

 appear, and the closely adherent peri- 

 carp forms the protection around the 

 embryo and endosperm. Naturally 

 ovules which have no integuments, 

 such as those of the Santalaceae and 



