GERMINATION, GROWTH, TlSStTE TENSION. 137 



the seed-coat during soaking, and the two large primary foliage- 

 leaves carried on the first internode (epicotyl) of the plumule. In 

 Runner the cotyledons are hypogeal, remaining below ground ; 

 while in French Bean they are epigfeal, being carried above ground 

 by the elongation of the hypocotyl the region of the young plant's 

 axis which lies between the root proper and the insertion of the 

 cotyledons. 



Most seedlings are epigeal, and it is easy to prove e.g. by making 

 Indian ink marks on the axis of the very young seedling and noting 

 the position of these marks at a later stage that the hypocotyl 

 grows rapidly in length, carrying up the cotyledons and the plumule. 

 Epigeal cotyledons sooner or later turn green on reaching the light ; 

 they are larger, thinner, and more like foliage-leaves than in the 

 case of hypogeal cotyledons, which do not turn green (unless they 

 happen to be exposed to light) and which soon shrivel up instead of 

 persisting and growing. Note that in Phaseolus the first two 

 foliage-leaves are simple and heart-shaped and stand opposite each 

 other ; while the later foliage-leaves are compound with three 

 leaflets, and arise singly from the stem. 



In the Garden Pea (Pisum sativum) the transparency of the coat 

 enables one to see clearly in the soaked seed the hilum, micropyle, 

 and radicle, all lying in the same line, with the tip of the radicle 

 pointing to the micropyle ; the cotyledons are hypogeal, and the 

 earlier foliage-leaves resemble those in Broad Bean seedling, but 

 the uppermost leaflets of the later leaves are developed as 

 tendrils. 



175. Sunflower Seed and Seedling. Get "seeds" of 

 this plant, also flower-heads of different ages, and note that 

 the " seeds " are in reality one-seeded fruits, or achenes, 

 each being formed from the ovary of one of the flowers in 

 the flower-head. The hard shell is not seed-coat, but peri- 

 carp or fruit- wall. The upper parts of the flower fall off 

 after fertilisation has occurred, leaving a ring-like scar at 

 the broad upper end of the achene the hole often seen at 

 the narrow end is (obviously) not the micropyle, but is 

 simply due to the breaking of the achene from the disc of 

 the flower-head. 



Soak some achenes in water for a few days, and open one 

 or two to examine the seed that lies inside ; the shell (peri- 

 carp) is readily split open along the edge. Note that the 

 seed is attached by a fine short stalk to the inside of the 

 shell at the pointed end. Eemove the thin seed-coat, and 

 note the radicle, the flat oval cotyledons, and the small 

 plumule. 



