CHAPTER LXXXVI 







KINDS OF PLANTS 



501. Higher and lower plants. We often speak of a given 

 kind of plant or animal as being higher or lower than another 

 kind. What we usually have in mind in making this distinction 

 is the fact that some organisms are simpler and others more 

 complex in structure. A dandelion is higher than the taller 

 willow, and the willow is higher than the pine, for the same 

 reason that we consider all three of these plants higher than 

 a fern or a seaweed. 



Complexity of structure has to do with the number of dif- 

 ferent parts or organs. Physiologically this corresponds to a 

 greater division of labor. We may compare three plants to 

 see the general differences as to structure and specialization 

 of functions. 



On the vegetative side the Spirogyra cell may be considered 

 a complete individual. It is capable of getting from the sur- 

 rounding water all that it needs to keep it alive, at any portion 

 of the surface. In a moss plant we may already see a division 

 into rootlike part, stemlike part, and leaflike part. The photo- 

 synthesis is carried on in one part of the plant ; the 'absorption 

 of water and salts is carried on in another part. The stem is a 

 connecting organ that holds up the leaves and also transports 

 (or, rather, transmits) materials between the two other organs. 

 In the bean plant we see a more complex root, with several 

 kinds of cells (tissues), a more complex stem and a more com- 

 plex leaf, both having several kinds of cells. The protective 

 layers of cells are different from the supporting (mechanical) 

 tissues ; these in turn are different from the conductive tissues 

 and from the photosynthetic tissues. 



477 



