266 



MORPHOLOGY 



(l'epcromia) there are sixteen free nuclei in the embryo sac. These 

 nuclei show no polarity, and a large number of the nuclei enter into the 

 formation of the large fusion nucleus, which in this case is the result of 

 multiple fusion. This same general condition has been found also in 



Juglans (walnuts), 

 U I mus (elms), the 

 aroids, etc. ; but 

 probably in all 

 these cases two or 

 four megaspore 

 nuclei are in- 

 volved. 



Nutritive mech- 

 anism. — The nu- 

 tritive mechanism 

 of the embryo sac 

 is varied and some- 

 times complex. 

 In all cases there 

 is an enlargement 

 of the sac, which 

 encroaches in 

 every direction 

 upon the adjacent 

 tissue of the nucel- 

 lus, which is thus 

 used as a nutritive 

 tissue. In some 

 cases, notably 

 among the Sym- 

 petalae, there is organized about the sac a definite nutritive jacket, 

 which obtains food from the surrounding tissue and from which it enters 

 the embryo sac. In other cases the antipodal end of the sac extends 

 into the tissue beneath (chalaza), sometimes becoming conspicuously 

 tubular and prolonged. In still other cases tubular extensions of the 

 sac are put out in other directions, especially into the heavy integu- 

 ment from the micropylar end. Occasionally all of these methods of 

 nutrition are combined, resulting in a complicated and very efficient 

 nutritive mechanism. 



Fig. 594. — Mature female gametophyte of lily within the 

 anatropous ovule, showing the egg apparatus (e, the e«g; the 

 two other cells the synergids) directed towards the micropyle (m), 

 the three antipodals at the other end of the sac, and the two 

 polar nuclei in contact in the center. — After Coulter. 



