THE FLOWER. 147 
that the matter takes place in nature. When a pollen-grain 
falls on the moist and viscous surface of the pistil, which we 
shall soon describe, and which bears the name of stigma, it ex- 
pands slowly, appearing to absorb its humidity ; the interior mem- 
Pee ea 
. Fee econ velope. ql 
Pollen of the Holy- port 
ock, first Envelope. ‘ollen of Wheat. 
Of Peibee aa OF Gantio. on  Phiox. of the Sedan 
brane becomes gradually extended and protruded through one or 
two pores in the exterior coating, in the form of delicate tubular 
protrusions, which lengthen by degrees, and end by forming real 
tubes, called pollen-tubes. 
The length of these tubes varies considerably ; they attain in 
certain cases many hundred times that of the pollen-grain, which 
gave it birth. This prodigious lengthening evidently ot p 1 
from a mere elongation of the internal membrane of the pollen- 
grain, but is the result of an actual growth in this membrane. 
The pollen-tube is nourished and grows, that is to say, it vegetates ; 
So that, leaving the stigma, it penetrates into the tissues, which it 
is intended to traverse. We shall have occasion to return to the 
pollen-tube when we speak of fecundation in the next section. 
Although pollen-grains are almost always free and distinct, there 
are some plants in which these grains are joined together, and 
often very closely. In the Orchidacee (Fig. 203), the pollen is 
gathered together into masses, sometimes almost pulverulent, with 
loosely-cohering granules; sometimes it is formed of numerous 
small angular masses, joined together by means of glutinous 
L 2 
