37 



Lecture IX. Summary . 

 (with regard to what has developed leaves.). 



Internal anatomy . Highest type of chloroplasts formed in 

 highest plants. Nat^^re seems to have heen experimenting in the 

 Algae. 



Leaf-form. Lowest form — unicellular, first — Protoccoccus. 

 Then filamentose, then expanded type. Natural order of evolution. 

 A plant to become m\ilti cellular must hecome filamentous, expanded, 

 or show evolution of an internal atmosphere. Adaptation of form 

 to external environment in lower forms; in higher also accommodation 

 must be made to the internal atmosphere as wellas outward surroundings, 



Direction of growth . 

 Lowest plant more or less horizontal ; in higher liverworts we have 

 the approach to verticality in stem. Evidence from Paleontology, 

 especially may be workejz^/d out witht he conifers. A perfect 

 series can be made out between eordates leaf and the needle upon 

 one side and the ginkgo on the other. 



A leaf is a mean between extremes ; the r^jsult of forces 

 acting in opposite ways, light and food supply against need for 

 protection. Ulva an ideal of what a leaf should be if developed 

 from a condition of light alone. 



