THE EMBRYO-SAC. 



405 



VJf 



is constructed as in Honotropa, except that the oosphere is less 

 deeply inserted. The antipodal cells are not visible ; in their 

 place is a strongly -refractive sub- 

 stance, in which lie, in fact, three 

 nuclei, recognisable, however, with 

 great difficulty. The pollen-tube 

 can, more easily than in Mono- 

 tropa, be traced to the synergidae ; 

 the changes which the synergidae 

 undergo are the same. Two nuclei, 

 moreover, are again found in the 

 fertilised oosphere. Endosperm is 

 in general not formed in the 

 Orchideae. 



In default of Monotrojm and of 

 Orchidaceae transparent ovules for 

 investigation are provided by various 

 Gesneraceae, and, above all, the 

 large-flowered Gloxinia hybrida of 

 gardens. The ovule, having only 

 one integument, is so far trans- 

 parent that the egg-apparatus is 

 clearly visible. It shows the two 

 synergidae, and the, in this case 

 flask-shaped, oosphere. In some 

 cases two oospheres can be present. 

 The embryo-sac in its upper part is swollen, but narrows 

 suddenly below ; the antipodal cells in the lower end are 

 not distinguishable with certainty. It is comparatively easy to 

 obtain an insight into the structure of the egg-apparatus by means 

 of sections through the ovary of species of L ilium or Narcissus. 

 As the ovules in the ovary cells of these plants are arranged quite 

 radially, longitudinal sections of ovules can be obtained by cutting 

 cross-sections of the ovary. The sections should be cut from fresh 

 ovaries, and examined in 5 per cent, sugar solution, and a con- 

 siderable number should be examined in order to obtain the 

 required structure with certainty. 



Emltryo-Sac and Fertilisation in Torenia. One of the most 

 favourable plants for the study of fertilisation is, however, the 

 Scrophularineous plant Torenia asiatica. This stove-evergreen 

 from the East Indies is now cultivated very generally in gardens, 



FIG. 147. OrtMt patient. Recep- 

 tive ovule, os, egg-apparatus ; ii-, 

 inner, ie, outer integument; I, air- 

 cavity. The other letters are as in 

 the previous figures ( x 240). 



