Metaphyta Lilium. 1 7 3 



spore- wall and later in the centre. The cellular tissue 

 (nucellus) surrounding the ovospore meantime disappears, 

 becoming absorbed by the endosperm, 1 which takes its place 

 morphologically and physiologically. 



The development of the embryo. The fertilised ovum 

 soon segments into a short string of cells, one extremity 

 being fixed to the end of the ovospore next the micropyle, 

 the other end of the string lying free in the endosperm. 

 This row of cells forms the FlG> ^_ VMa tricolor , (Sac hs.) 

 pro-embryo. The proximal 

 cells of the row, i.e. those 

 next the micropyle, become 

 the suspensor, and the 

 single distal cell becomes 



the embryO proper. Posterior end of embryo-sac : e, wall ; S, 



_,, j i f cavity of the embryo-sac ; A", A", yoijng 



1 lie development OI endosperm cells ; pr, protoplasm of the 



this distal cell into the 



embryo is of some importance, and it is at this stage that 

 we meet with the first great difference between monoco- 

 tyledons and dicotyledons. The account here given is true 

 for both types up to a certain stage, when the points of 

 difference will be emphasised. 



The distal cell first undergoes subdivision, so that eight 

 daughter cells are formed, which form a blastoderm (to use a 

 convenient zoological term). These eight cells next become 

 tangentially divided into eight concavo-convex cells exter- 

 nally and eight small tetrahedral cells internally (fig. 91). 

 The external concavo-convex cells become dermatogen, or 

 the layer destined to give rise to the epidermis ; the inter- 

 nal tetrahedral cells form the embryonic periblem, or 

 fundamental tissue, and plerome, or fibro-vascular tissue. 

 In the dicotyledons the embryo becomes flattened and 

 heart-shaped, and shows a differentiation into two lobes, the 

 cotyledons, or primary leaves, between the bases of which 



1 In some few plants it remains, and is then known as perisperm. 



