THE EMBRYO-SAC. 401 



a pale yellowish parasite, found chiefly under beech and fir trees. 

 This plant is so favourable for the otherwise difficult investigation 

 of the embryo-sac that no pains should be spared, if possible, to 

 obtain it. 1 It flowers in July and August, and must be examined 

 fresh, as in alcohol it becomes brown and opaque. The plant 

 bears carriage very well, and can be preserved healthy for a very 

 long time in a glass of water. Supplies may in this way be 

 obtained from a distance. 



If it is necessary to keep Monotropa in alcohol, this latter 

 should have sulphur dioxide added to it, since in pure alcohol the 

 object becomes dark brown. For this purpose sulphur dioxide gas 

 should be passed into the alcohol. This operation will take about 

 a minute if we pour a little 80 per cent, sulphuric acid over sodium 

 sulphate in a suitable vessel, and conduct the gas evolved into the 

 alcohol. To each 100 c.c. alcohol about | gram sodium sulphate 

 should be allowed. The object remains in this sulphurated alcohol 

 for about twenty-four hours, and can then be transferred to pure 

 alcohol. With Monotropa agree the various species of Pyrola, or 

 " winter- green," excepting that their ovules are smaller. About a 

 dozen species of Pyrola, all hardy herbaceous perennials, can be 

 readily enough cultivated in gardens, either from seeds or division 

 of the roots, selecting for the purpose a shady border, with a sandy 

 peat soil. 



The cross-section through the under part of the ovary shows 

 this to be four-celled, five-celled in Pyrola. The placentae are 

 strongly swollen, and bear on their surface very numerous, slender, 

 closely packed ovules. The two halves of the placenta in each 

 cell are separated to some little distance by a radial groove. In 

 the upper part of the ovary these grooves extend to the centre of 

 the ovary, and there coalesce. We see then four strong pairs of 

 placentae, each placed near the centre of a septum, and which 

 appertain to two adjoining loculi ; the pairs are easily separated 

 from one another with the needles. We obtain ovules for study 

 by removing a portion of the wall of the ovary with the forceps, 

 and stripping off with needles the ovules from the placenta thus 

 exposed. W T e place them in pure water, or in 3 per cent, solution 

 of sugar, in which the ovules remain longer unchanged. If we 

 take our material from an oldish flower, in which the stamens 



1 It is hardly likely to be regularly obtainable in England, so that the 

 alternative plants referred to must be relied on for material. [Eo.] 



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