STAGES IN PLANT EVOLUTION 51 



compared. In such a view the great difference between 

 the highly specialized cells x, px, b, &c., and those of 

 the main mass of ground tissue p becomes apparent. 



Even in the comparatively simply organized groups 

 of the Equisetales and Lycopodiales the differentiation 

 of tissues is complete. In the mosses, and still more 

 in the liverworts, it is rudimentary; but they grow in 

 very damp situations, where the conduction of water 

 and the protection from too much drying is not a difficult 

 problem for them. As plants grow higher into the air, 

 or inhabit drier situations, the need of specialization of 

 tissues becomes increasingly great, for they are increas- 

 ingly liable to be dried, and therefore need a better flow 

 of water and a more perfect protective coat. 



It is needless to point out how the individual cells of 

 a plant, such as that figured in figs. 19 and 20, have 

 specialized away from the simple type of the protococ- 

 coid cell in their mature form. In the young growing 

 parts of a plant, however, they are essentially like proto- 

 coccoid cells of squarish outline, fitting closely to each 

 other to make a solid mass, from which the individual 

 types will differentiate later and take on the form suit- 

 able for the special part they have to play in the eco- 

 nomy of the whole plant. 



To trace the specialization not only of the tissues 

 but of the various parts of the whole plant which have 

 become elaborate organs, such as leaves, stems, and 

 flowers, is a task quite beyond the present work to 

 attempt. From the illustrations given of tissue struc- 

 ture from plants at the two ends of the series much 

 can be imagined of the inevitable intermediate stages in 

 tissue evolution. 



As regards the elaboration of organs, and particularly 

 of the reproductive organs, details will be found through- 

 out the book. In judging of the place of any plant in 

 the scale of evolution it is to the reproductive organs 

 that we look for the principal criteria, for the reproduc- 

 tive organs tend to be influenced less by their physical 



