388 TEXTBOOK OF BOTANY 



develop from the embryos of seeds that matured within the fruit. Some- 

 times during excessively wet weather the embryos of corn, wheat, and 

 other grains germinate almost immediately after maturity. In these wet 

 seasons one frequently finds embryos continuing growth inside the ears 

 of standing corn in September, and wheat and oats sprouting in the 

 shocks on the field during midsummer. 



Extreme cases of embryo development are found in seeds of such 

 plants as Christmas holly, orchids, and mangrove. The embryo of the 

 holly is merely a minute spherical body of cells when the fruit turns red 

 in December. This clump of cells grows very slowly during a period of 

 8 to 12 months. The germination of the seed is further complicated be-, 

 cause the developing embrvo cannot break out of the hard fruit coat 

 until the coat is partially decayed. 



Orchid seeds also consist of comparatively few cells and do not germi- 

 nate unless thev are artificiallv supplied with sugar or unless fungi digest 

 the insoluble food in the surrounding substrate. The balloon-shaped 

 green seedlings become readily visible onlv after a lapse of 6 months, and 

 at the end of two years seedlings in many species are less than an inch in 

 height. 



The seeds of mangrove, on the other hand, germinate while the fruit 

 containing them is still attached to the tree. The hypocotyl emerges from 

 the seed and fruit and becomes more than a foot in length and a half 

 inch in diameter. Then this great hypocotyl, with a tiny plumule at its 

 apex and a root primordium at its heavier basal end, drops from the tree 

 into the mud below, and rapid development of plumule and roots fol- 

 lows (Fig. 172). 



The embryos of most plants, however, are fully developed within the 

 ovule, but the seed will not germinate immediately. Seeds of these plants 

 germinate only after a longer or shorter period of time has elapsed, even 

 though placed in environments with water, oxygen, and temperature 

 conditions ordinarily suitable for growth. 



From embryo to seedling. Some of the processes in embryos during 

 germination have been described in Chapter II and Chapter XXIX. If 

 several seedlings are watched as they emerge from the soil, certain strik- 

 ing differences in their growth are evident. 



When the castor bean germinates, the lengthening of the hypocotyl 

 raises the cotyledons, endosperm, and broken seed coats above the soil 

 surface. The cotyledons rapidly expand in area and slough off the remain- 

 ing endospenn tissues and seed coats. The cotyledons are thin blade-like 



