6.5 Reading #3
: Sexual Reproduction in Gymnosperms
A gymnosperm's lifecycle is characterized by alternation of generations, just like that of
angiosperms. The male and female gametophytes are found in the cones of conifers like pines, and
the green, leafy portion of the plant is called the sporophyte (Figure 121). The male cones are small
and situated in the lower part of the tree, while the female cones are larger and situated near the top
of the tree. Because the pollen is shed and blown by the wind, this arrangement makes it difficult for
a gymnosperm to self-pollinate.
176
Figure 121.
The lifecycle of a conifer. Pollen from male cones blows up into upper branches, where it
fertilizes female cones. Examples are shown of female and male cones
91
.
Male Gametophyte
A male cone has a central axis on which bracts, a type of modified leaf, are attached. The
bracts are known as
microsporophylls
(Figure 122) and are the sites where microspores will develop.
Inside the microsporangium, the microspores grow. Microsporocytes, which divide through meiosis
to create four haploid microspores, are the cells that make up the microsporangium. The microspore
undergoes additional mitosis, yielding the generative nucleus and the tube nucleus. When the male
gametophyte (pollen) reaches maturity, it is released from the male cones and is then carried by the
wind until it lands on the female cone.
Female Gametophyte
The female cone also has a central axis on which bracts known as
megasporophylls
(Figure
122) are present. Megaspore mother cells can be found in the megasporangium of the female cone.
Four haploid megaspores are produced after the meiotic division of the megaspore mother cell. While
the other megaspores divide to create the remainder of the structure, one of them divides to create the
multicellular female gametophyte. The archegonium is a structure that houses the female
gametophyte.
91
(credit “female”: modification of work by “Geographer”/Wikimedia Commons; credit “male”: modification of work
by Roger Griffith
|
