292 Beer, Development of the pollen grain and anther of some Onagraceae. 
"When tetrads at tbis stage are treated witb 10 % KOH tlie 
callose wall is dissolved and tbe protoplasts, eacli surrounded bv 
tlie undissolved pollen membrane, are set free. 
We may conclude from these reactions tbat tbe young 
pollen wall is composed of a pectic substance. The remains of 
tbe primary sporogenons cell wall, whicb also gives tbe reactions 
of a pectic body, can still lie distinguisbed at tbe peripbery of 
tbe tetrad. Altbougli tbe association between tbe newly deve- 
loped pollen membrane and tbe special-niotber-cell wall is so 
close tbe demarcation between tlie two is always sbarp and 
tbere is never a gradation of one into tlie otber. AVbere tbe 
callose wall abuts npon tbe pectic membrane it is denser and 
refracts tbe ligbt more strongly tban tbe rest of tlie wall pro- 
bably forming tbere a „Grenzbäuteben“ in Strasbnrgers 
sense. 
Tbe facts sliow tliat altbongb tbe pollen wall is at first 
deposited in close contact witli, and probably in actual attacb- 
ment to, tbe special-motber-cell wall it is not derived from a 
transformation of tbe innermost lamella of tbis callose wall but 
is directly secreted as a pectic layer by tbe cell-protoplast. It is 
equally certain tliat tlie plasmoderma 1 ) is not bodily transformed 
into tlie pollen wall bnt tbat tbis is deposited upon tbe surface 
of tbe plasmoderma, as Strasburger bas sliown in otber casese. 
As soon as tbe pollen membrane becomes' sligbtly tbicker 
it separates readily from tbe callose layers and is tlien clearly 
recognised as an independent structure. Tbe young pollen gram 
is a blimtly triangulär, basin-sbaped structure witb tbe concavity 
of tbe „basin“ directed towards tlie centre of tbe tetrad. It 
measures about 19 to 20 fi across its broadest surface in 0. biennis. 
At tbe apex of tbe tliree angles of tbe pollen grain tbe wall is 
extremely tbin. Tbe protoplasm fills tbe cell-cavity and con- 
tains a considerable quantity of starcb (Figs. 15 and 16). 
In pollen grains wbicb are a little older [mearuring about 
22 to 24 [/ across in 0. btennis 2 )] tbe wall lias tbickened con- 
siderably and a mucilaginous material bas been developed at 
tbe three ungles of tbe cell at tliose spots wbicli previously were 
thin (Fig. 17). Tbis mucilage gives tbe reactions of a pectic 
substance and appears to be derived from tbe growtb and pliy- 
sical alteration of tlie pollen wall at tliese points. Tbe little 
plugs or discs of mucilage continue to enlarge and soon bulge 
so far witliin tbe cell tbat tbey overlap tbe unswoln pollen 
wall on eacb side. 
9 I use tliis term as the eq ui valent of tlie german „Hautscliicht“. Tlie 
word was suggested for this purpose by Strasburg er and first used by 
Stevens in his paper upon „Grametogenesis and Fertilization in Albugo“, 
(Bot. G-azette XXXII. 1901. p. 92.) 
2 ) In stating tbe size of the pollen grain I liave always taten the 
measurement across the broad face of the grain from the tip of an inter- 
stitial body to the outer surface of the wall immediatly opposite. 
