278 



ELEMENTS OF GENERAL SCIENCE 



in small clouds like yellow dust When the pollen grains 

 alight on the stigmatic end of the pistil, important changes 

 follow. From the inner wall of the pollen grain there develops 

 a tubular outgrowth which grows down through the stigma 

 and style into the ovule and so reaches the vicinity of the 

 egg. While the pollen tube has been growing down into the. 

 ovule two male cells have developed within it. One of these 

 may unite with the egg cell, thus producing the fertilized egg. 

 Each male* cell is capable of fertilizing the egg in the ovule. 



306. The embryo plant. After fertilization the egg of the 

 plant like the egg of the frog proceeds at once to grow. 



r A B 



FIG. 130. Diagram to show development of young plant from the egg 



A, entire pistil; B and C, development of seed; D, seedling plant; pol., pollen 

 grain; p. t., pollen tube; ov., ovule; sac, embryo sac; emb., embryo; sd., 



mature seed 



Through cell division and enlargement one region develops 

 into the root tip, another into the stem tip, and others into 

 one or more leaves. In most cases, when the young plant 

 has developed these different regions, the wall of the ovule 

 becomes hardened and the embryo becomes dormant. This 

 is the mature seed. It may lie dormant for a short or a long 

 period and then grow. 



307. The seedling. When seeds of plants are placed in favor- 

 able situations, moisture is absorbed by the young plantlet in- 

 closed within the seed coats, arid its growth is soon continued. 



