THE SPRUCE FIR 271 



It is not necessary to follow the early development 

 of the ovule in detail, for up to a certain point it 

 grows in very much the same way as the ovule of an 

 Angiosperm. If the reader will refer to the develop- 

 ment of the ovule (see pp. 118 and 180), as described 

 in the Wallflower or the Lily, he will understand what 

 goes on here. Here, as in Angiosperms, the important 

 changes all go on in the nucellus or middle part of 

 the ovule. The archesporium arises from a cell just 

 below the epidermis of the nucellus. The arche- 

 sporium is a single cell to begin with. It divides by 

 a tangential wall ; from the upper part tapetal cells are 

 formed. The lower part divides up into a row of 

 three cells, and it is usually the lowest of the three 

 which becomes the embryo-sac. Both the epidermis 

 and the tapetal cells undergo a great many divisions 

 by tangential walls, so that the embryo-sac comes to 

 be buried deep down in the nucellus, beneath a thick 

 mass of tissue. 



The embryo-sac at first, like any other cell, has a 

 single nucleus, which soon divides up repeatedly, 

 until there are a large number of nuclei in the proto- 

 plasm lining the cell-wall of the sac. These nuclei 

 are at first free, but after a time cell-walls are formed 

 between them, cutting up the parietal layer of proto- 

 plasm into a layer of cellular tissue. The cells thus 

 formed continue to grow and divide until the whole 

 embryo-sac is filled with tissue. The growth of the 

 sac still goes on, until by the time of fertilisation, in the 

 latter part of June, it is about J of an inch in length. 

 The growth and cell-division of the internal tissue 

 keep pace with the enlargement of the embryo-sac. 



