Lecture XXVII. 249 



Spores of two kinds (microspores and megaspores) on their ap- 

 propriate sporophylls are found in each flower. The microspores 

 (pollen-grains) germinate as they are about to leave the microspor- 

 angia (pollen-sacs), but the gametophyte is only formed if they reach 

 the receptive pad (stigma) on the megasporophyll (carpel). There 

 the microgametophyte (pollen-tube) develops parasitically and trans- 

 mits to the ovum of the megagametophyte its sperm. The 

 megagametophyte also develops parasitically but without leaving 

 the sporangium (ovule) in which the megaspore originated. After 

 fertilisation the oosperm becomes the embryo enclosed in the 

 megagametophyte which is still surrounded by the sporangium. As 

 in the Buttercup this seed contains the young sporophyte, the rem- 

 nants of the two gametophytes and a portion of the parent sporo- 

 phyte. 



The salient differences between the Hyacinth and the Buttercup 

 consist in the arrangement and number of the parts of the sporo- 

 phyte. Under these categories may be mentioned the suppression, 

 or delay in the development, of one cotyledon, which leads to the 

 lateral position of the growing point, the parallel arrangement of 

 the conducting tracts in the leaves and their " scattered " arrange- 

 ment in the stem. In the number of the floral leaves we have 

 another striking difference which is general among the Monocotyle- 

 dons like the Hyacinth, and the Dicotyledons like the Buttercup. 

 In the former the number of the floral leaves in each category is usu- 

 ally some multiple of three, while in the latter it is more usually a 

 multiple of four or five. Cohesion of the leaves, whether in the 

 perianth or among the carpels, is found in both groups. 



PRACTICAL WORK 



Sketch a complete plant of wild Hyacinth, showing leaves, bulb, roots and 

 flowering stem. 



Make a transverse section of the flowering stem. Sketch and show the 

 position of the conducting tracts. 



Sketch a flower from above, showing the carpels, stamens and perianth. 



Make a special sketch of the carpels alone. 



Prepare a longitudinal section of the flower and draw it, showing the at- 

 tachment of the various floral leaves. 



Locate the dissolved pigment in the vacuoles of the cells of the perianth. 



Make a transverse section of the ovary. Show in your sketch the three 

 carpels and the attachment of the ovules. 



