234 Practical Plant Biology. 



which is bent back on itself where it enters the ovule. The ovule 

 consists of a central egg-shaped mass of cells the nucellus 

 enveloped in two coverings, the integuments. The integuments 

 do not close over the top of the nucellus but leave a passage lead- 

 ing to it. This passage is called the micropyle. Owing to the 

 bend in the stalk the micropyle is directed towards the base of the 

 stalk. A conducting tract runs from the base of the nucellus down 

 the stalk and joins one of the two conducting tracts in the adaxial 

 side of the ovary wall. One of the cells in the axis of the nucellus 

 elongates and increases in size. It divides into four, and while one 

 of these, the embryo-sac, begins to grow rapidly, the remaining 

 three are crushed, as are the other cells surrounding the embryo- 

 sac. The growing embryo-sac to all appearance puts them under 

 contribution for its support. 



When the supremacy of the embryo-sac has been clearly estab- 

 lished its nucleus divides into two and the segments, moving to 

 opposite ends, subdivide again. The four nuclei thus produced 

 divide again and so there come to be eight nuclei in the sac four 

 at each end. So far no cell-walls have been formed and the sac 

 is a ccenocyte. Now, however, three of the nuclei in the base of 

 the sac, i.e. the end furthest from the micropyle, become separated 

 off by cell-walls. These are called the antipodal cells. Two of 

 the nuclei in the other (micropylar) end apply themselves against 

 the wall at that end and becoming the centres of two masses of 

 protoplasm are called the synergidse, while against them rests a 

 third nucleated mass of protoplasm the ovum. The remaining 

 two nuclei (polar nuclei), one from each end of the sac, move to 

 the middle and coming into contact with one another ultimately 

 fuse together and form the fusion-nucleus. 



The lower outermost stamens are the first to ripen and dehisce, 

 thus exposing their pollen on the open anther-lobes. This occurs 

 before the styles and stigmas of the carpels of the flower have 

 emerged above the closed anthers of the upper stamens. 



Ripening proceeds from below upwards and when the uppermost 

 anthers are opening the stigmas of the carpels come to maturity 

 and are covered with the sticky fluid. 



Various small insects, flies, beetles, and even bees visit the 

 flowers of the buttercup to gather the honey produced in the 

 pockets at the base of the petals. In visiting the freshly opened 

 flowers they rub against the ripe anthers and become dusted with 

 pollen which they carry away with them as they leave the flower. 

 When they alight on a more mature flower, the under sides of their 

 bodies rub off some of the pollen thus acquired on to the stigmas 



