522 MR. ir. MARSHALL WAKD ON 



Before proceeding to describe the changes which result in the 

 formation of the embryo-sac, I propose to dwell on certain pecu- 

 liarities of cell-growth, division, and arrangement shown by such 

 an ovule as this, since it appears probable that many phenomena 

 common to such structures are related more or less simply to the 

 distribution of the cells. 



Disregarding the integuments and funiculus, a comparison of 

 the figures shows that we have in thenncellus a body growing up- 

 wards and outwards from its base (the so-called chalaza) ; and a 

 remarkable connexion is evident between the arrangement of the 

 cells and the distribution of growth in the organ. For instance, 

 in PL XVII. fig. 7 the diameter of the nucellus has somewhat 

 evenly increased in a direction perpendicular to the long axis ; 

 and to effect this, the subepidermal cells have each become divided 

 by a wall parallel to the periphery. This is well seen in PI. XVII. 

 fig. 8, which is drawn from a section across the middle of an ovule 

 like PI. XVII. fig. 7. And another fact is illustrated at the same 

 time : the layer of cells thus cut off outside at once divide by 

 radial walls (that is, walls perpendicular to the periphery of the 

 organ), and so by doubling their number fill up the larger space ; 

 and it will be noticed that a similar division is made by the epi- 

 dermis, which has a larger surface as it is pushed out by the 

 widening mass beneath. 



So, too, the integuments must provide for increased surface 

 by more walls in vertical and radial planes. As the nucellus in- 

 creases in volume, growing forwards by additions at its base, and 

 by cell-divisions in planes perpendicular to the long axis, its form 

 changes from almost cylindrical to more or less egg-shaped, with 

 the narrow end free ; and a corresponding increase in the number 

 of cells occurs. It is not perhaps possible to state clearly the re- 

 lations between the form of the ovule and the distribution of the 

 cells ; but we may note some peculiarities in this connexion*. The 

 most obvious fact is the general perpendicularity of the cell-walls 

 to the walls they cut ; this is true especially of the epidermal 

 cells, but applies to others as a rule when first formed. 



A second peculiarity is that in the completed ovule the cells of 

 the nucellus are, except the axial series, arranged in rows of which 

 the directions radiate from the lower third of the central mass to 



* See also a paper by Sachs in Wiirzburg Arbeiten, " Anordnung der Zellen in 

 jungsten Pflanzentheilen." 



