386 A TEXTBOOK OF BOTANY [Ch. VIII, 6 



aspect of a seed tells whether it is still alive or not, or what 

 percentage of a given quantity is alive, the purchaser of seeds 

 is at the mercy of a dealer unless he can himself make test of 

 viability. For such tests various methods have been devised, 

 the most simple and chrect of which is that of placing a 

 given number in folds of blotting paper kept wet, dark, and 

 well aerated, and noting the percentage which germinates. 



6. The Cycle of Development from Seed to Seed 



Having studied the six primary parts of plants -with respect 

 to their structures and functions, it remains to consider 

 their successive appearance in that cycle of development 

 through which every individual passes. It is possible to 

 break the cycle for study at any desired point, but in prac- 

 tice we may best start mth the germinating seed. The facts 

 having already been considered in detail, we can best review 

 the subject in a way to bring out its general principles. 



The seed contains a well-formed embryo plant, provided 

 with stem, I'udiments of root and h\id, and cotjdedonarjr 

 leaves, all enwrapped with a store of food substance inside 

 protective coats. In germination the seed alDsorbs water, 

 swells, and bursts the coats ; the stem pushes forth its 

 lower end, whicli grows over geotropically dovtmward 

 and enters the ground. Meantime its tip is developing a 

 root, which, on contact with the soil, puts forth many root 

 hairs, whereby it absorbs osmotically a sufBciency of water. 

 No sooner is the root secure in the ground than the stem 

 makes growth bendings which first withdraw the cotyledons 

 from the seed coats, and then lift them geotropically upward 

 until they open out to the light on the tip of the vertically 

 straightened stem. Meantime the whole plant is swelling 

 rapidly in size through absor])tion of water, and turning 

 green over stem and leaves by formation of the chlorophyll 

 so essential to its future welfare. Thus the fully germinated 

 EMBRYO now stands rooted in the ground and erect in the 

 sun, to which it spreads a large surface of chlorophyll. In 



