190 PLANT COMMUNITIES 



to the oak tree we may compare the egg cell before fertilization with 

 a bare area. It represents a potential plant ; but it will never develop 

 until something, the male gamete, comes into it from the outside. 



We are using birth here only in the sense of a beginning or an 

 origin. Therefore, we may take the fertilized egg (a single cell) 

 as the beginning or birth of the new individual that is to develop 

 into an oak tree. Corresponding to this on the bare area we would 

 have first a single crustose lichen. In order that these young 

 organisms may grow they must both receive something more from 

 the outside. The fertilized egg must receive food and the young 

 community must receive plants. The fertilized egg now develops 

 into an embryo, and the young community develops into a lichen- 

 moss community; the embryo develops into a seedling, and the 

 lichen-moss community develops into a perennial herbaceous plant 

 community; the seedling develops into a shrub, and the perennial 

 herbaceous plant community develops into a shrub community; 

 the shrub develops into a sapling, and the shrub community develops 

 into a xeric tree (oak-hickory) community. These are mere pro- 

 gressive steps in the development of these two organisms. The 

 sapling now matures into a tree, and the xeric tree community 

 matures into a mesic tree (beech-maple) community. 



The mature oak tree, which is a representative of the second type 

 of individual, is a community of individuals of the first type, and it 

 reproduces by giving off single individuals of the first type (cells or 

 gametes). The plant community, on the other hand, is a com- 

 munity of individuals of the second type, and it reproduces by 

 giving off single individuals of the second type (plants, usually in 

 seeds). As to death, there seems to be no reason why a tree should 

 ever die unless it gets diseased or meets with an accident. Some of 

 the big trees of California have lived for several thousand years and 

 there is no reason to suppose that they will not keep on living unless 

 they become diseased or meet with an accident. The same thing 

 may be said of a plant community. It may die from disease or from 

 an accident, but otherwise it will continue to live indefinitely. It is 

 a little hard at first to visualize the phenomena concerned with this 

 third type of organism only because we have not been used to doing 

 so. It is necessary to keep in mind the bigness of these organisms 

 and of the phenomena concerned and to learn to think in terms of 

 bigness. For example, some organisms of the first type (bacteria) 

 may complete their life cycles in a half-hour, for some of those of the 

 second type (a tree) it may take a hundred years, but for those of the 



