THE BEGINNINGS OF PLANT LIFE 29 



its stem, possessing a central system of cells, 

 with a protective outer layer of other cells, and 

 so something akin to the hark__pr skin of the 

 higher plant forms appears. Fig. 20 shows an 

 alga possessing this first differentiation in stem 



Fig. 18. A many-celled globular plant (Volvox 

 globator) which swims about in ponds. Actual 

 size about ^ of an inch 



structure. Advancing structure may readily be 

 traced, although all algse 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 



