\*The Plant Kingdom 



ONE OF THE first attempts to classify all 

 known organisms was made bv the great 

 Swedish biologist, Carolus Linnaeus; and by 

 the time of his death in 1778, Linnaeus was 

 able to describe almost 10,000 species. Since 

 then, however, the number of known organ- 

 isms has increased enormously. Today, in 

 fact, almost 900,000 animals and 400,000 

 plants are recognized. No one person, of 

 course, can ever hope to learn about all these 

 species. But one person can succeed in mak- 

 ing a broad survey of the field and can readily 

 learn how to recognize the principal types 

 of plants and animals. 



The aim of modern taxonomy is to classify 

 organisms according to their evolutionary 

 relationships; but this represents an exceed- 

 ingly difficult task. The fossil record (paleon- 

 tology) provides considerable data, though 

 many of the pages and even whole chapters of 

 this record are incomplete and other parts 

 have been badly scarred bv time. Often it is 

 necessary to judge relationships on the basis 

 of indirect evidence, derived from a study 

 of the comparative morphology, physiology, 

 embryology, and biochemistry of the organ- 

 ism in question. Consequently, it is not sur- 

 prising to find that biologists may disagree 



592 



on certain questions and that classification 

 may keep changing as more evidence accu- 

 mulates. 



THE PLANT AND ANIMAL KINGDOMS 



There are many similarities between plants 

 and animals and these bear witness to a 

 fundamental kinship. In fact, all basic studies 

 on metabolism, growth, responsiveness, re- 

 production, heredity, and so forth, in plants 

 and animals generally, tend to show this 

 fundamental kinship. Also many similarities 

 of cellular structure are revealed bv micro- 

 scopic studies. All in all, the evidence indi- 

 cates that plants and animals both originated 

 from a common ancient ancestral stock 

 (Chap. 10). Consequently it is not surprising 

 to find that no single criterion, bv itself, can 

 be used to make a sharp distinction between 

 the plant and animal species. 



Typically, of course, plants are green and 

 display the holophytic type of nutrition. But 

 there are many exceptions to this rule (Chap. 

 10). Generally, plant cells possess a cellulose 

 wall, but this is lacking in some lower forms 

 (for example, Euglena); and cellulose is pos- 

 sessed by the cells of one animal group, the 



