188 



STRUCTURAL BOTANY 



ledonous and dicotyledonous Classes differ. These 

 differences are important, and among them we shall 

 find one of the most constant points of distinction 

 between the two divisions. 



The nucleus of the fertilised ovum, consisting of the 

 united male and female nuclei, divides, and this is 

 followed by the division of the ovum as a whole. The 



first two walls formed 

 are transverse. Of 

 the three cells thus 

 produced, the one 

 next the micropyle 

 (which we will speak 

 of as thelowest) under- 

 goes no further divi- 

 sions, but often grows 

 to a great size, as in 

 the embryos shown 

 in Figs. 89 and 90, 

 and then forms a 

 temporary store of 

 food for the embryo. 

 The uppermost cell, 

 instead of forming 

 nearly the whole of 

 the embryo, as in our 

 dicotyledonous type, 

 here only gives rise to one part, namely, the single 

 seed-leaf or cotyledon, which in this case forms the 

 free end of the embryo. 



The middle cell of the three divides by further 

 transverse walls, forming, after some longitudinal divi- 

 sions have taken place, the groups marked st and r in 

 Fig. 90. 



The group of cells marked st is a very important one, 

 for from the cells on one side of this group arises the 

 growing point of the stem, which in Monocotyledons 



FIG. 88 Longitudinal section of ovule of 

 Scilla (a plant allied to the Lily) after 

 fertilisation, p.t, pollen -tube; em, 

 embryo ; e, e, endosperm nuclei still free 

 in the protoplasm of the embryo -sac. 

 Other parts as in Fig. 83. Mag. 55 diam- 

 eters. Somewhat diagrammatic. (R. S. ) 



