DEVELOPMENT OF OVULES 



359 



All Angiospermous ovules are supplied by a vascular bundle 

 which runs through the funicle, but terminates at the chalaza, 

 except in those rare instances (e.g. Myrica) where the nucellus 

 is not in intimate contact with the integument and hence 

 provided with a special vascular supply. 



The ovule invariably arises on the placenta as a small mound 

 of thin-walled cells which represents the future nucellus (Fig. 

 207, A and D, I). Around the base of this the integuments 

 subsequently develop as ring-like up- 

 growths, the inner being formed before 

 the outer (II-IV). If a third integument, 

 or aril, be present (as in the Spindle-tree, 

 Euonymus), this is the last to arise, and 

 only becomes conspicuous after fertilisa- 

 tion. 



All ovules are at first erect. The 

 anatropous form, characteristic of the 

 majority, is due to excessive growth on 

 one side of the chalazal region of the 

 rudiment, so that the latter gradually 

 becomes curved till the micropyle points 

 towards the placenta (cf. Fig. 207, D). 

 In the case of campylotropous ovules 

 the mature form is due to even more 

 extreme asymmetrical growth of the 

 chalazal region (Fig. 207, E). Stages in 

 the development of anatropous ovules 

 can be readily studied in transverse 

 sections through young ovaries of the 

 Poppy. 



In the case of the stamens or microsporophylls the leaf-like 

 character is not so obvious as that of the carpels. The lamina 

 is represented by the connective which bears the microsporangia 

 (pollen sacs) on its margin, i.e. in the same position as the 

 ovules. Clearer evidence of the foliar nature of the stamen is 

 afforded by cases in which the connective is developed as a flat 

 petal-like structure (as in the Pondweeds, Potamogeton, Fig. 

 208, B, Co.), or in which stipule-like outgrowths arise from the 

 bases of the filaments (as in the Onions, Allium, Fig. 208, C). 



FIG. 20 8. Various forms 

 of stamens. A, Tulip 

 (ordinary form). B, 

 Pondweed (Potamo- 

 geton), showing the 

 enlarged connective 

 (Co.). C, Onion (Al- 

 lium), with stipular 

 outgrowths from the 

 base of the filament. 

 a., anthers ; /., fila- 

 ments. 



