206 Practical Plant Biology. 



length. Their outer ends seem embedded in the scale while their 

 pointed inner ends project free from it and are directed towards 

 the axis of the cone at the side of the attachment of the scale to 

 the cone. This pointed end is often tipped with brown. 



The cones which are two years old are found just below the 

 growth of the preceding year. They are brown and woody, but 

 have not grown further in size. Their stalk is straight and the 

 scales are widely divergent from one another. By this time the 

 ovules have become seeds and some of them are very probably 

 already shed. The seeds are larger than the ovules and they are 

 brown in colour. They are covered with a hard coat to which is 

 attached a brownish semi-transparent wing formed from some of 

 the tissue of the upper surface of the seed-scale which has adhered 

 to the seed and detached itself from the seed-scale. This wing 

 delays the fall of the seed when shed from the cone, and makes it 

 easily wafted about by air-currents. 



The ovule on the seed-scale consists of a small mass of tissue 

 called the nucellus. Round this, a collar of tissue develops which 

 grows up over it and almost completely covers it, leaving only a 

 small passage leading down to the nucellus. The covering is 

 called the integument and becomes the seed-coat; the passage 

 penetrating the integument and leading to the nucellus is called 

 the micropyle. The micropyle is directed towards the axis of the 

 cone and slightly away from the middle line of the upper surface 

 of the carpel. One of the cells centrally placed in the growing 

 nucellus elongates parallel to the axis of the nucellus and divides 

 transversely into four. One of these four grows rapidly, while it 

 crushes and partially absorbs the remaining three and other sur- 

 rounding cells. This enlarging cell is known as the embryo-sac. 

 As we shall see, it is in reality a megaspore. The megaspore con- 

 tinues to enlarge so that its protoplasm is reduced to a lining ad- 

 hering to the wall. Meantime its nucleus divides and this division 

 is succeeded by divisions in its products. The numerous nuclei 

 so formed are distributed in the layer of cytoplasm lying against 

 the wall of the megaspore or embryo-sac. Cell- walls now begin to 

 be formed and separate the superficial nuclei into cells. The un- 

 enclosed nuclei continue to divide and the cytoplasm increases in 

 bulk, gradually invading the central vacuole. With the continuance 

 of cell-division the whole cavity of the embryo-sac is ultimately 

 filled with tissue, which is called endosperm. During this time 

 the nucellus is considerably encroached upon and distorted by the 

 growth of the embryo-sac. It is finally reduced to a conical cap 

 of cells covering the top of the endosperm. The apex of this cap 



