DEVELOPMENT OF PLANTS 



359 



reaching the megasporangia, in the spring, develop only a short 

 tube cell during the first season and not until about the first of 

 July of the following season are the male and female gametes 

 mature and ready for fertilization. The seeds are matured during 

 the two following seasons, over three years after the appearance 

 of the strobilus. Accordingly, three stages in the development 

 of the strobilus of megasporophylls may be seen on certain spe- 

 cies of pine in the early summer — very small ones that received 

 the microspores in the spring, larger ones in which fertilization 

 and the formation of the embryo is being effected and older stro- 

 bili in which the seeds are approaching maturity (Fig. 252). 



Fig. 259. Renewal of growth of the embryo. At the left a seed has been 

 cut across to show the relation of parts during the early growth. The 

 integument has been ruptured by the swelling of the seed and the protrusion 

 of the root which is curving down into the soil. The cotyledons, c, remain 

 in the seed absorbing the food from the gametophyte or endosperm, g. At 

 the right a later growth with the cotyledons partially withdrawn from the 

 seed after the absorption of its food. 



When the conditions are favorable the embryo renews its growth. 

 The root is pushed out through the micropyle and bends down 

 into the soil, the hard integument often being cracked open by 

 the swelling of the cells. The cotyledons remain within the seed 

 until they have absorbed all the food from the gametophyte, 

 when they are withdrawn and become erect (Fig. 259). 



