GERMINATION, GROWTH, TISSUE TENSION. 139 



youngest foliage-leaves are trifoliate or three-lobed ; those formed 

 later are simple, narrow, and spine-tipped. 



In Brazil "nut" (really a seed) the hard shell is the seed-coat; 

 the minute cotyledons occupy one end of the embryo, the root being 

 at the other end. The greater part of the embryo consists of the 

 swollen axis (hypocotyl). The two cotyledons and the plumule can 

 be seen in a section examined with the microscope if the section 

 has been cut in exactly the right place. 



IV 7. Castor Oil Seed and Seedling. In the seed 

 note the hard and usually mottled black or brown seed- 

 coat, bearing at one end an appendage (aril) which absorbs 

 water readily and becomes soft when the seed is soaked. 

 Place a seed in hot water, and note that air-bubbles arise 

 from beside the aril, which lies just outside of the micro- 

 pyle. Eemove the coat, dissect the seed contents, and make 

 transverse and longitudinal sections ; note the embryo 

 which lies in a cavity in the middle of the white oily 

 endosperm and consists of two very thin flat cotyle- 

 dons (pressed against the endosperm but easily separated 

 from it by means of a knife point), the small plumule 

 between the bases of the cotyledons, and the radicle 

 below the cotyledons and reaching the surface of the 

 seed at the micropyle-and-aril end; with care the em- 

 bryo can be dissected from the endosperm ; the cotyle- 

 dons show a distinct midrib with veins arising from it 

 on either side. 



On germination the hard seed-coat splits into three 

 valves, the hypocotyl emerges at the other end of the seed 

 and, after the radicle has grown into the soil, elongates and 

 pulls up the seed into the air ; the elongating hypocotyl is 

 hooked at the top ; rootlets grow out usually in four 

 regular longitudinal rows from the top of the radicle ; the 

 endosperm becomes swollen and gradually thins out to 

 a papery film covering the outer (lower) surfaces of the 

 two cotyledons, which meanwhile grow larger ; then the 

 shrivelled film of endosperm is ruptured by the cotyledons, 

 which spread out in the air (the hypocotyl becoming 

 straightened) as heart-shaped leaves with short stalk and 

 prominent veins between the cotyledons the plumule is 

 plainly seen. 



