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Beer.— Studies in Spore Development . 
whilst the protoplast is not in contact with them. In 1908 Tischler ( 21 ) 
observed another and most striking example of this phenomenon. In the 
pollen-grains of Mirabilis Jalctpa the protoplasmic contents degenerate 
and shrink to a scarcely noticeable quantity, and yet notwithstanding 
this the exine of these grains continues to grow very considerably in 
thickness and extent. For some time past I have been examining a 
large number of spores and pollen-grains belonging to many species of 
plants in order to find, if possible, other examples of membranes which 
are able to continue their growth without the direct co-operation of the 
protoplasm. I have, up the present, found no other such striking cases 
as those of Selaginella , Isoetes , Oenothera, or Mirabilis , but a careful 
examination of so large a number of different spores could not fail to 
bring to light many interesting details which supplement our present 
knowledge of the subject. 
I have already given a description in these pages 1 of two of these 
spores (viz. Helminthostachys and Riccia ), and I now propose to add an 
account of some other pollen-grains and spores which I have had under 
observation. I intend to deal with the pollen-grains of Ipomoea in the 
present part. 
I. Ipomoea. 
The only account of the finer structure of the pollen-grains of 
a species of Ipomoea is the very short description given by Strasburger 
in 1889 ( 17 ). After having furnished a minute account of the spinous 
pollen of several species of Malvaceae, Strasburger devotes a few lines 
to the pollen-grains of Ipomoea coccinea , Moench., which appeared to him 
to be constructed quite after the manner of the Malvaceous type. 
I have examined the pollen-grains of Ipomoea purpurea , Roth., 2 in some 
detail, and as I find that these differ in several respects from the Malvaceous 
type of pollen, I will begin these ‘ Studies ’ with a description of them. 
My material was fixed partly in Flemming’s solutions and partly in 
chrom-acetic mixture without osmic acid. So far as possible I have 
checked my results by comparison with living material examined in 
o-6 % NaCl solution, but the opacity of the structures did not render this 
method a very satisfactory one in the present case. 
The pollen mother-cells of Ipomoea usually form two or sometimes three 
longitudinal rows in each pollen-sac ; they are each surrounded by a wall 
which gives the reactions of callose and also of pectose, and they include 
a rather large nucleus (about 14 /x in diameter) which contains, as a rule, 
a single large nucleolus and a loose reticulum of fibres. The tapetal cells, 
1 Ann. of Bot., vol. xx, April, 1906, pp. 177-186, and Ann. of Bot., vol. xx, July, 1906, 
pp. 275-291. 
2 This plant is also known as Pharbitis hispida (Choisy). 
