Fossil plants, Vol. 2 : $b A text-book for students of botany and geologySeward, A. C. (Albert Charles)
Science
Fossil plants, Vol. 2 : $b A text-book for students of botany and geology
Seward, A. C. (Albert Charles)
Paleobotany
Dr Scott has also described a strobilus containing microsporangia
partially enclosed by a rudimentary integument. It is, however, of
considerable interest to find a partial development in the case of a
male flower of an integumentary outgrowth, which it would seem could
only be of real functional importance in the female shoot.
It is important to notice that specimens of a second species of
_Lepidocarpon_, _L. Wildianum_, are recorded from Lower Carboniferous
beds of Scotland, a fact which points to a considerable antiquity for
this seed-bearing Lycopodiaceous type[668].
The most important question to consider in regard to _Lepidocarpon_
is—are we justified in applying to the integumented sporangia the term
seed? The megaspore was not set free as it is in recent Pteridophytes,
such as _Azolla_ and other genera with which _Lepidocarpon_ may be
compared; it was on the other hand retained in the sporangium, as
may sometimes happen even in recent species of _Selaginella_ (cf.
fig. 131, D). Moreover, the megaspore is characterised by a thin
enclosing membrane in contrast to the thick coat of a spore which
is destined to be shed. The peculiar slit-like form of the micropyle
is a distinguishing feature, but this may be readily explained as
a convenient form in the case of a radially elongated sporangium.
The absence of an embryo, though a distinguishing feature of
_Lepidocarpon_, cannot be held to be a serious obstacle to the use of
the term seed; in recent Cycads the embryo, as Scott points out, may
not begin to develope until the seed has been shed. It is possible that
the seeds of _Lepidocarpon_ were not pollinated on the parent plant.
The lesson which this extinct type teaches, is that certain
Lycopodiaceous plants of the Palaeozoic era had reached an important
stage in the evolution of a seed. The morphological essentials of true
seeds had been acquired; but we do not know the biological conditions
under which pollination and fertilisation were effected. Another point
of considerable interest is the value of this discovery as an argument
in favour of the view that some Gymnosperms are derived from Lycopod
ancestors. Leaving the general question until later, it may at any rate
be stated that in _Lepidocarpon_ we have a demonstration of the fact
that the Lycopodiales were not always distinguished from Gymnosperms
by the absence of seeds. There are certain features in _Lepidocarpon_
shared by the seeds of Araucarieae[669] which may well mean something
more than mere parallel development in two distinct phyla of the
plant-kingdom[670].
ii. _Miadesmia._
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