THE BEGINNINGS OF PLANT LIFE 29 



its stem, possessino- a central system of cells, 

 with a protective outer layer of other cells, and 

 so something akin to the bark or skin of the 

 higher plant forms appears. Fig. 20 shows an 

 alga possessing this first differentiation in stem 



Fig. 18. A many-celled glolmlar plant { ro/vox 

 globator) which swims about in ponds, .\ctual 

 size about ^Vi of ^^ inch 



structure. Advancing structure may readily be 

 traced, although all alga; are still of lowly form, 

 for even with our last example, each disc on its 

 stem is but a cluster of simple thread-like cells all 

 radiating from the one point. 



So we may trace a gradual rise from the lowest 

 to the highest forms of life, and learn to recognise 



