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A TEXTBOOK OF THEORETICAL BOTANY 



STIGMA 

 STYLE 



UMENTS 



EGASPORANGIUM 

 (NUCELLUS) 



MEGASPORE 

 (EMBRYO SAC) 



thick-walled and fleshy, and is known as the nucellus. The nucellus in its 

 turn has been enclosed within one or two coverings called the integuments, 

 which are of uncertain morphological nature, but are probably related to the 

 cupules that surround certain Pteridosperm seeds. All the above elements 

 together constitute the ovule. Among the Gymnosperms the ovules are 

 attached directly, without any further protection, to the female sporophylls. 

 In the Angiosperms the ovules in their turn are enclosed in a case, the ovary, 

 which forms a part of the organ called the carpel, a structure of great 



importance because it forms the 

 fruit, which is the distinguishing 

 mark of this, the highest group of 

 the Seed Plants. The carpel prob- 

 ably represents a development of 

 the megasporophyll (Fig. 649). We 

 shall return to a further discussion 

 of seed structure and behaviour in 

 Volume II. 



Although the megaspore and the 

 megasporangium have thus under- 

 gone a marked change in relation- 

 ship and also in structure during 

 the evolution of the seed, the micro- 

 spore and the microsporangium have 

 remained relatively unaltered and are 

 649. — Diagrammatic section through an recognizable in the Flowering Plants 



angiospermic carpel with enclosed ovule, j^ jj j ^ ^ jj 



showmg the various parts. cio 1. ^ ^ ^ ^ 



sac respectively. 



The evolutionary process seems to have taken place at a very early 

 geological date and in more than one group of plants. Some members of the 

 Lycopodiales seem to have arrived at the seed habit quite independently, 

 notably the genus Lepidodendron, but they did not succeed in evolving into 

 Flowering Plants, possibly owing to some deficiency in their anatomical 

 structure. 



On the other hand, we find in the Carboniferous rocks a group of plants 

 showing close similarities with the Ferns, which bore seeds upon their fronds. 

 The name Pteridospermae, given to this group, indicates their intermediate 

 position. They are the most primitive seed plants in the Pteropsida and 

 they never themselves produced the special aggregation of sporophylls which 

 we call a flower, though they probably represent the stock from which the 

 Flowering Plants have been derived, as the latter are certainly Pteropsid rather 

 than Lycopsid in their affinities. 



Biologically the evolution of the seed was an immense gain. Within its 

 protective covering the embryonic plant develops in safety ; by mechanical 

 developments of this same covering it is successfully dispersed and by the 

 stores of reserve food which the seed contains it is tided over the critical 

 and dangerous period of its establishment as an independent plant. When 



Fig. 



