456 



SPERMATOPHYTES (SEED PLANTS) 



tube is eating its way through the nuceUus to the female 

 gametophyte. 



Male Gametophyte. — The male gametophyte forms within 

 the pollen grain and its tube. At the time of pollination the male 

 gametophyte commonly consists of four cells — two prothallial 

 or vegetative cells, a generative cell, and a tube cell. At least one 

 of the prothallial cells usually disintegrates and disappears early 

 in the development of the gametophyte. This is the condition 

 of the male gametophyte when the pollen is carried to the ovulate 

 cone. Upon reaching the ovulate cones the pollen grains fall 

 down to the base of the scales in the region of the ovules, and 



Fig. 405. — Seed structures of the Pine. A, a mature ovulate strobilus 

 with scales spread apart to allow the seeds to escape. B, a view of the inner 

 side of a scale, showing the two seeds when mature. The wings of the seeds 

 are a part of the scale and did not develop from the ovule. C, section through 

 a pine seed, showing the female gametophyte {g), embryo (e), and seed coat (w) . 



some lodge at the mouth of the micropyles, where they are caught 

 in a drop of a mucilaginous secretion and drawn in close to the 

 tip of the nucellus. In this position the pollen grains begin to 

 develop tubes, which by means of an enzyme dissolve the nucel- 

 lar tissue, using it as food and at the same time making a way for 

 themselves. Cold weather finally checks the growth of the 

 pollen tubes, and the male gametophytes now rest over winter. 

 Early the next spring the pollen tube resumes its growth toward 

 the archegonia, and the generative cell passes into the pollen tube 

 and divides, forming two cells, one of which divides and forms the 

 two sperms whicli now have the pollen tube as a passageway to 

 the archegonia. The sperms reach the archegonia about the 



