The evolution happening at different times around the world corresponds to differing environmental conditions, and the rate at which evolutionary advantageous mutations spread around the globe. The transition first occurred in North America and Western Europe, where it was completed by the beginning of the Late Permian, while in Asia and Australasia palaeophytic forms continued into the Early Triassic.
In a paper by Knoll (1984) it is stated "wherever fossiliferous sequences are complete enough, demonstrably paleophytic and mesophytic assemblages are separated by intermediate floras that contain elements of both groups".
These evolutionary changes have been brought about by the drying out of the planet, which spelled the end of the warm, wet swamp forests so prevalent in the Carboniferous. Plants and their seeds and spores had to adapt to be able to retain water within them to prevent dehydration and death.
This led to water loving plants such as the Lycopods and Sphenopsids greatly reducing in size, becoming small shrubs, with ferns and seed ferns being the dominant species.
It has been recognised for well over a 100 years (Weiss, 1877) that a great evolutionary step for the plant world was in the Late Permian with the arrival of gymnosperms which created many new plant species, including the conifers and gingkos which would proceed to diversify into prominance in eras following the end-Permian.
While on the whole the plant world has not undergone a mass extinction like the animals at this time, rather a adaptive transition, there is a slight decrease in the total number of plant species. This is due to the extinction of one plant group, the Cordaites, at the end of the Permian. These were mangrove-like swamp trees, a reconstructive sketch of one can be seen in the image to the left.
There was an extinction in only one other major plant group, with the Glossopteris floras which were dominant in Gondwana being replaced by the seed fern Dicroidium as the climate dried.
On the whole there was not a mass extinction of the plants as is seen by the fauna at the time, rather a gradual transition from Palaeophytic to Mesophytic forms brought about by the initiation of new plant groups responding to changing environmental conditions.