Page 15 - 11-Bio-9 Kingdom plantae

9. Kingdom Plantae eLearn.Punjab

Overtopping

The dichotomously branched aerial portion of the stem showed unequal branching. Some branches
remained short while others grew and expanded at a much faster pace. All these branches grew in
different planes. Such an unequal development of various branches is called overtopping.

Planation

Next important step was the arrangement of unequal dichotomies in one plane. This process is
termed as planation.

Fusion/Webbing

Overtopping and planation was followed by a process known as fusion or webbing. The space
between the overtopped dichotomous branches was occupied by a sheet of parenchyma cells
which connected these branches forming a flat lamina or leaf blade type of structure, having many
dichotomously branched veins (Fig. 9.13).
During the course of evolution fusion of the vascular strands resulted in net or reticulate venation
pattern. The process of evolution of leaf was very slow and gradual which completed in more than
15-20 million years.

LYCOPSIDA

The plants of Lycopsida have sporophytes differentiated into roots, stem and true leaves. The
leaves are small and single- veined, they are also called microphylls. The arrangement of leaves is
spiral or opposite. The sporangia develop singly on the upper side of the sporophylls, which may
or may not be arranged to form strobili (Fig. 9.14)

The sporophyte may have sporangia of one kind as in Lycopodium or of two kinds i.e., microsporangia
and megasporangia as in Selaginella (Fig. 9.15).

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