522 



A TEXTBOOK OF BOTANY 



[Ch. XIII 



specialized placental or supporting growth for the ovules, 

 in which case the cone is a many-carpelled single flower; 

 or the outer may be simply a subtending bract and the inner 

 composed of two sporophylls, joined edge to edge, and de- 

 rived from an otherwise suppressed axillary bud, in which 

 case the cone is morphologically a cluster. The megaspore 

 germinates internally and becomes 

 filled with prothallus (endosperm), in 

 which occur the egg cells. Above and 

 around each egg cell occur certain 

 cells which are the rudiment of an 

 archegonium. When ready for pol- 

 lination, the young cone is upright, 

 with the scales apart; the pollen, 

 brought by the wind, slips down the 

 scales to the ovules, which have secreted 

 water drops in the gaping micropyles. 

 These drops later dry up, thereby 

 drawing the pollen into the micropyle 

 Ovule of near the nucellus. The grains germi- 

 nate slowly, and the tube carrying the 

 sperm cell to the egg cell does not 

 The coats nucellus e ff ect fertilization until the following 



(shaded), embryo sac, . . 



endosperm, embryo on a spring, the cones taking two seasons to 

 suspensor, archegonia, ma ture. When pollination is accom- 



with distinct neck cells, . . 



and pollen grain and pushed, the scales of the cone close 

 tube, are all obvious, together, and the cone itself turns over 



(From Sachs.) ° 



to hang downward, thus to remain 

 until the seeds are ripe. The fertilized egg cell divides on 

 the side away from the micropyle into four tiers of cells (Fig. 

 367) ; one of the intermediate tiers then elongates in the en- 

 dosperm, which is digested by enzymes as they grow. This 

 is the suspensor, which develops the embryo proper at 

 its end. The embryo when grown is polycotyledonous, and 

 lies in the midst of the endosperm, which is absorbed by the 

 cotyledons in germination. Ultimately the cone opens, and 



Fig. 367 

 a Gymnosperm in longi- 

 tudinal optical section 

 diagrammatic. 



