EXTERNAL CONDITIONS ON PLANT LIFE. 169 



base for purposes of respiration. Its flattened cone is the 

 result of the bad nutrition. The land form, dropping into its 

 native dry earth habitat, does not develop the knees and 

 enlargement, because its oxygen supply is ample. Its nourish- 

 ment, too, is sufficient, so that it will have as a result of these 

 normal, favorable conditions, the tall, sharp, cone-like form 

 natural to the pine family. Doubtless the cypress has been 

 millions of years adapting itself from its dry land conditions 

 to its watery surroundings, and the most interesting fact in this 

 connection is its readiness to fall back into its old habit of 

 growth, and even in the first generation on land to lose every 

 trace of these wonderful acquisitions. 



There are many other plants which have acquired equally 

 remarkable organs in the same way. Some of the mangroves 

 of the Florida coast such as Avicennia nitida and Lagnncnlaria 

 raccinosa, growing in the ooze between tide- waters, have developed 

 vertical roots which project out of the mud by hundreds under 

 each tree and are exposed to the air at every low tide. These 

 organs aerate the plant as the knees aerated the cypress. 

 They are much more highly organized, being covered on the 

 parts above the mud with great numbers of large, open 

 lenticels. 



It is not necessary for us to go so far from home to find 

 numbers of plants which have adapted themselves, through 

 special aerating organs, to a change from dry land to water 

 growth. One of the most common is Dccodon vcrticillatns, 

 which develops over the surface of all its roots an extremely 

 thickened, corky, air-holding layer. A similar development 

 covers the branches when they happen to drop into the water. 



Let us now see how plants are modified when they lack 

 water. If the supply is inadequate, the plant makes an 

 attempt to conserve the little that it has. As the surface of 

 ordinary plants allows large quantities of water to escape, by 

 transpiration, and as this loss is largely proportioned to the 

 amount of surface exposed, such plants usually lessen this 

 surface by making the leaves smaller and thicker, or by losing 

 them entirely, by shortening the branches and by consolida- 

 tion generally. As the light and the heat of the sun increase 



