CONCLUSION 235 



water by transpiration through the stomata or 

 pores. Plants growing in salt swamps also have to 

 limit their transpiration, to avoid having to take 

 in too much salt water. Probably this was the 

 explanation in the case of the Palaeozoic Lycopods. 



The system of secondary thickening, i. e. 

 the power of developing unlimited wood and 

 bast by the activity of a layer of dividing cells, 

 the cambium, adding new wood to the inside 

 and new bast to the outside, is an excellent adapta- 

 tion for enabling plants to grow into trees. We 

 are accustomed to associate secondary growth 

 chiefly with highly-organised Seed-plants, the 

 Conifers and the Dicotyledons. In Palaeozoic 

 times, we find that all the groups of Vascular 

 Cryptogams had adopted secondary growth it 

 is no new advance, but has existed from the 

 earliest times, whenever there was need for it. 



Sometimes quite special devices connected 

 with the secondary tissues were anticipated 

 in the Palaeozoic Flora. In some Conifers, at 

 the present day, there are water-conducting 

 vessels in the pith-rays which run out through the 

 wood between pith and bast. The vessels in 

 the rays serve as a water-connection between 

 the inner and outer layers of wood. Just the 

 same mechanism is found in the wood of Lepidoden- 

 dron; the two parallel structures must have been 

 evolved quite independently. 



