362 Tue Microscope. 
have found twenty one that burst thus readily. A much larger 
number swell slowly, discharging their contents as the expansion 
progresses, so that examination is not feasible after two or three 
minutes of immersion. A few kinds seem unaffected ky water. 
Most grains can be examined with advantage in undiluted 
sulphuric acid. Points of structure come out in this medium 
which cannot be seen on the dry grain, nor on it when immersed 
in water. The examination was made with uniform promptness 
after immersion, as it was found that in many cases there was 
with the lapse of time, a slowly progressive change of form. 
Generally, pollen grains swell slightly in sulphuric acid. But 
some show a decided shrinking. 
The extine and intine are differentiated by sulphuric acid. 
The pollen of Tilia Americana affords the finest illustration of this. 
Saponaria and Lychnis are also good examples. 
Certain groups of closely related families have pollen that 
becomes red or brown in sulphuric acid. Thus one group in the 
Polypetalee comprising the Pink, Purslane and Mallow families ; 
another under the Apetalee comprising the Four-o’clocks and 
Goosefoots. Pastinaca and Cicuta, it seems, are isolated cases ; so 
is Cirsium lanceolatum. The pollen of Apocynum turns light brown ; 
that of Sagittaria, blackish. In all other cases, it is implied, 
there is no appreciable change of color. 
Both the Pinks and the Goosefoots have spherical pollen, with 
from ten to thirty round perforations in the extine; and, as 
stated, both turn red in sulphuric acid. Amaranthus has a simi- 
lar pollen, but it does not usually thus turn red. Let me here 
ask whether this similarity of pollen grains does not point to, or 
suggest, a common ancestry of the Pinks and Goosefoots, now 
classed so far a part in the natural system of plants. 
The Borrages have some of the smallest pollen grains, which 
are dumb-bell-shaped, and endure well both water and acid, 
hardly changing size or form in either medium. The grains of 
the Parsley family are also inclined to be dumb-bell-shaped, but 
they are much larger, and further differ from the Borrages in 
that they have from three to six perforations around the con- 
stricted part of the grain. Some of the largest pollens are those 
of Malva moschata (148), Gnothera biennis (1291), Iris versicolor 
(oval, 70 by 184), Lilium Philadelphicum (oval, 52 by 89), 
Erythronium Americanum (67 by 93); Anacharis (now Elodes) 
