Sperm-Cells in Asclepias Cornuti , Decaisne . 125 
or less interrupted. The membrane of the pollen-grain is 
described 1 as made up of two united coats, and the cavity 
filled with spherical granules of nearly uniform size. These 
granules were not often found in the pollen-tubes, nor were 
the tubes branched or jointed. The figures accompanying 
this paper are excellent. Brown, without doubt, considers 
the cells of the pollinium as true pollen-grains. 
In 1840 Jussieu figured the pollinia with pollen-tubes pro- 
jecting, and considered the pollinia as masses of pollen-grains. 
Schleiden (1849) also considers the pollinia as masses of pollen, 
clothed with a special membrane. 
In his work on the Microscope (1853), Schacht gives 
directions for the microscopic study of the flower of A. syriaca , 
and describes (Chapter 4) and figures correctly for the first 
time (Fig. 15 ) the pollinium mother-cells for this plant. 
I find no mention of Schacht’s work by later writers on 
this subject. Schacht, however, derives all of the pollen- 
grains of a single anther-locule from a single ‘ primary 
mother-cell,’ which he figures (Fig. 21 a) 2 . Engler (1876) 3 
states that this is not true for Asclepias , though it is the mode 
of development in Albizzia lophantha , Benth. The writings 
of Sprengel (1793) and others I will only mention, as they are 
concerned with fertilization in the old sense of the term, i. e. 
pollination 4 . 
In 1 885 appeared Wille’s paper on the development of the 
pollen-grains in Angiosperms. The first part of this paper 
deals with the mode of formation of the membrane or coats of 
pollen-grains. In the Orchidaceae he traces ‘ a successively 
retrograde developmental process. 
1. Free pollen-grains in Cypripedium. 
2. Free tetrads with a new membranous structure on the 
exterior in Epipactis. 
1 Loc. cit., p. 724. 
3 The same writer in his later paper (1860) refers to the cells of the pollinium 
as pollen-grains. 
3 p. 298. 
4 Meyen (1838 N , Delpino (1865), Hildebrand (1866), Robertson (1886-7), and 
Ule (1897). 
