Botany lecture notes

Types of Stelar Systems and its Evolution in Pteridophytes and Higher Plants with PPT


types of steles in pteridophytes

Stelar Evolution in Vascular Plants
(Origin and Evolution of Stele in Pteridophytes and Higher Plants)

What is stele? What are the components of stele?

Ø  Stele is the central cylinder or core of vascular tissue in higher plants.

Ø  The stele consists of xylem, phloem, pericycle and medullary rays and pith if present.

Ø  The term ‘stele’ was for the first time used by Van Tieghem and Douliot in 1886 in their ‘Stelar Theory’.

What is ‘stellar theory’?

Ø  Proposed by Van Tieghem and Douliot in 1886.

Ø  Major highlights in stellar theory are:

$.  The stele is a real entity and present universally in all axis of higher plants.

$.  The primary components of stele are xylem and phloem.

$.  Tissues like pericycle, medullary rays and pith are also the components of stele.

$.  ‘Stelar theory’ also says that the cortex and the stele are the two fundamental parts of a shoot system.

$.  Both these components (stele and cortex) are separated by the endodermis.

$.  In higher vascular plants (Pteridophytes, Gymnosperms and Angiosperms), the leaf traces are large, and it appears that they play an important role in the vascular system of the axis.

$.  The whole set-up of leaf traces appears as a composite structure in these plants.

$.  Such composite structures do not remain within the limits of stellar theory of Van Tieghem and Douliot.

What are the different types of steles in plants (Pteridophytes and higher plants)?

Ø  On the basis of ontogeny and phylogeney, there are THREE broad categories of steles in vascular plants.

Ø  They are:

(1).  Protostele

(2).  Siphonostele

(3).  Solenostele

Continue reading

Botany lecture notes

Complex Tissue System in Plants: Part 2 – Phloem – Structure, Components and Classification (with PPT)


cell types in phloem

Phloem
Structure, Composition & Classification of Primary and Secondary Phloem

What is phloem?

Phloem is a complex tissue system in plants. It is the food conducting tissue of vascular plants. Together with xylem, they form the vascular tissue system. The phloem composed of several types of cells among which some are living cells and some are dead. The term ‘phloem’ was introduced Nageli (1853) from a Greek word ‘phloios’ meaning ‘bark’. The ‘bark’ is a non-technical term describing all tissue outside the secondary xylem of the plant. Botanically the bark includes secondary phloem, cortex, primary phloem and periderm. The current post describes the structure, composition and classification of phloem.

Location of phloem in plants:

Ø  Usually, the phloem is situated external to xylem.

Ø  In leaves, the phloem is located on the abaxial side (lower side).

Ø  In some plants (members of Cucurbitaceae and Convolvulaceae), the phloem is present on both external and internal to xylem. Such a vascular bundle is called bicollateral vascular bundle.

Ø  Phloem present internal to the xylem is called ‘internal phloem’ or intra-xylary phloem.

Ø  Phloem located external to the xylem is called ‘external phloem’.

Continue reading