evolution of leaf and seed

hope well thanks for your great support in the previous segments of this blog so in this blog i would like to talk about evolution of leaf and seed about their definitions different things related to them all to the steps that took place during their evolution and all about this and the thing that i always ask if you want to know about anything related to biology so please let me know down in the comment section
so thing thing to talk about is what seed is so actually a seed is defined as a fertilized ovule
so now what is ovule so ovule is defined as an integumented indehiscent megasporangium 
integuments are specialized protective coverings around megasporangium that may vary in number 
steps involved in evolution of seed
seed plants predominate over non seeded vascular plants one of the most significant events in the history of land plants was the development of seed habit it was an important change in the reproductive system of the vascular plants that occurred about approximately 390 million years ago first complete appeared approximately 365 million years ago during late Devonian period all the seed producing plants are also know as the spermatophytes in them sperm means seeds and phytes means plants so spermatophytes means such plants that produces seeds the various steps that are involved in the evolution of seed habit are as following 
*evolution of heterospory
*retention and germination of megaspore with in the megasporangium 
*development of protective layers around the megasporangium 
*reduction to a single functional megaspore per megasporangium 
*development of an embryo sac with in the sporangium 
*modification of the distal end of megasporangium for pollen capture 
evolution of heterospory 
* primitive vascular land plants produced only one kind of spores a condition known as the homospory all groups of land plants up to pteridophyes are homosporous 
* during early stage of evolution some of the pant groups started producing two different types of spores the smaller ones called as the microspore and the other one that is the large one known as the megaspore and such a condition heterospory 
* the microspores produced inside microsporangium germinated to form male gametophyte or the microgametophyte
* megaspore germinate to form female gametophyte or megagametophyte
retention and germination of megaspore within the megasporangium 
* during the usual reproductive cycle in the heterosporous vascular land plants the megaspore used to be shed and dispersed soon after the formation in order to germinate into female gametophyte 
* in some of the plants such as selaginella the megaspore was not allowed to escape from the megasporangium immediately after its formation 
* in others megaspore was permanently retained within the megasporangium here with in the confined walls of megasporangium the megaspore than germinate to form the egg that has the female gametophyte 
development of protective layers per megasporangium 
some branches like the structure of sporophyte surrounding the megasporangium fused around megasporangium to form protective envelope or integument the megasporangium tightly locked by integuments that become totally indehiscent this important change led to the evolution and formation of ovule which is nothing but an integumented indehiscent megasporangium in such a way more of the protection was accorded to the egg containing apparatus in terrestrial environment
reduction to a single functional megaspore per sporangium 
each of the formed megaspore mother cell with in a megasporangium used to produce four functional mega-spores 
*if these megaspores germinate to produce four viable female gametophyte there may have been a competition for space and for food among the four gametophyte 
*early vascular plants adopted a new strategy such that only one megaspore was selected for the further development into a healthy female gametophyte while the remaining three were aborted 
development of an embryo sac with in the megasporangium 
only a single healthy megaspore retained with the megasporangium to form an egg containing female gametophyte called as the embryo sac 
modification of distal end of megasporangium for the pollen capture 
when most of the structural and functional changes leading to the development of seed habit were completed another important modification took place in the megasporangium which was now integumented indehiscent and permanently attached to the sporophyte the distal end of the megasporangium become modified for the purpose of capturing pollen microspore containing male gametophyte 
pollen after being trapped in the distal cavity of the megasporangium produced pollen tube which carried male gamete deep into the embryo sac to fertilize the egg forming a zygote which than form a embryo the megasporangium ovule after fertilization is transformed into a seed the integuments becoming the seed coat the seed offers maximum degree of protection to a developing embryo under the unfavorable terrestrial environment the evelopment and evolution of seed habit was a great success and a giant leap which ultimately enabled plants to colonize on land permanently
so these are some of the steps that took place during the evolution of seed 
leaf is the photosynthetic organ of plant
plants before evolution of leaf   
early vascular land plants did not have true leaves or roots they were small in size with dichotomously branched erect smooth aerial parts are equally strong subterranean anchoring and absorbing rhizome 
one of the very best example of plants before leaf is Rhynia such that it have naked stem without leaves 
types of leaves 
there are two important type of leaves that are 
such type of leaf which is small and has a single undivided vein vascular supply is known as the microphyll
evolution of microphyll
rhynia like plants started to form leaves as a small scale like outgrowth these outgrowths were not supplied with vascular bundles therefore they were not regarded as the true leaves lycopods were the first plant that formed the true leaves and roots  however in lycopods such that in lycopodium the leaves are small in size each of the leaf has a single undivided vein and are called as the microphyll 
large leaves having divided veins and veinlets with an expanded leaf blades or lamina are known as the megaphyll 
evolution of megaphyll
 it is suggested that the evolution of megaphyll started from a dichotomous branching system in some of the primitive ferns like plants approximately 350 million years ago
it is assumed that the evolution of a megaphyll includes a series of successive evolutionary steps this process of evolution was very slow and gradual because in like it is a fix rule that modification that we also know as the evolution takes a lot of time such that this evolution of megaphyll completed in about 15-20 million years these are the following steps involved in it 
it is defined as the unequal development of various branches 
during this step the dichotomously branched aerial portion of the stem showed unequal branching some of the remained short while others grew and expanded at a much faster pace all of these branches grew in different planes
in this step there was arrangement of unequal dichotomies in only one plane this process is thus termed as planation 
during this step of the evolution of leaf 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 of the dicthotomously branches veins 
during the coarse of evolution fusion of vascular strands resulted in net or reticulated venation pattern 
significance of leaf
leaves have increased surface area for the absorption of light thus caused increase in process of photosynthesis 
                thanks for reading                                          



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