294 Mr Corry, On the Development of [Nov. 27, 



with in considerable numbers and a distinct nucleus may be 

 detected. 



Very soon however by the aid of reagents, especially the 

 aniline color methyline-blue 1 , a delicate thin transparent hyaline 

 membrane or wall is found to clothe and to have been formed 

 all over its surface by the protoplasm, which has in some cases 

 where the preparation has been treated with alcohol slightly 

 contracted away from this wall. The membrane is however 

 exceedingly difficult to detect at this stage. This change takes 

 place simultaneously in all the special-mother-cells. The newly 

 formed cell, consisting of a very thin and delicate cellulose wall, 

 closely applied to the internal side of the pale yellow cuticularized 

 wall of the special-mother-cell by which it is surrounded, but 

 from which it may be made to contract away by means of alcohol, 

 enclosing protoplasm loaded with vacuoles and rendered dark 

 with minute granules, and a nucleus, is the equivalent of the 

 pollen-grain of other plants, and will in future to indicate this 

 feature be designated by the same title. The mode of formation 

 of the pollen in Asclepias is very different to that which is the 

 characteristic and prevalent type in the majority of Dicotyledons 

 or Monocotyledons; and, so far as our present knowledge extends, 

 exhibits in its entire details a perfectly unique, isolated and 

 peculiar case of development. The earlier stages are only to be 

 found paralleled in the single instance of Zostera, which affords 

 either the most primitive or most aberrant type of pollen-forma- 

 tion known. The later stages find . no. precise parallel in the 

 entire range of the vegetable kingdom. This is the more remark- 

 able since another member of the Asclepiadeae, viz. Periploca 

 graeca, exhibits according to Reichenbach, a type of pollen- 

 formation exactly comparable to that of the Orchid genera 

 Neottia and Epipactis 2 . 



Observations on the mode of development of the pollen in 

 Asclepias are fraught with extreme difficulty, and its history can 

 only be revealed by careful study of extremely thin transverse 

 and longitudinal sections. 



In many of the pollen-grains, especially when the flower was 

 fully mature, I was able by careful observation and by having 

 recourse to osmic acid of one per cent, strength and to staining 

 reagents such e.g. as Haematoxylon, Grenicher's carmine, and 



1 I owe the suggestion that I should make use of this staining reagent to my 

 friend Mr W. Gardiner, who has employed it largely in his researches on "The 

 continuity of the protoplasm in the motile organs of leaves." 



2 Dr S. H. Vines suggests that probably in Asclepias and likewise in Zostera 

 the phase of the special-mother- cells as it occurs in other plants is omitted, and 

 hence we get the marked departure from the normal types. On this view what 

 I have termed the special-mother-cells are really the last series of mother-cells 

 produced by repeated division of the single primary one. 



