YOUTH AND ADULT LEAVES 447 



leaves. Here again we see the relation of the youth form 

 to the conditions of its environment. 



The adult form, as we have seen in the case of sun and 

 shade leaves, may often be modified by externsl conditions 

 to a greater or less extent, or rather the production of the 

 youth form may be prolonged or reverted to. Gliick (1905) 

 found that in water 5 ft. deep Sagittaria produced extremely 

 long band leaves, and only a few floating leaves, while in 

 greater depths, as a rule, submerged leaves only were 

 formed. An adult plant transferred from shallow to deep 

 water starts to form leaves of the band type again. If a 

 harebell, with an erect stem bearing linear leaves, and about 

 to flower, is placed in low Ught intensity the flower buds 

 wither and side shoots with round leaves are produced. 

 If no flower buds have been formed the main axis may 

 proceed to the formation of round leaves. The whin 

 cultivated in moist air forms broader and softer leaf spines 

 and the formation of shoot spines is suppressed, though no 

 trifoliate leaves are produced. For further details Goebel, 

 Arber, and the monographs of Gliick (1905) and Diels 

 (1906) should be consulted. 



§ 10. Seasonal Changes, Protection, and Rhythm 



Annuals. — Some plants complete their existence in a 

 single season or may run through several generations in 

 a summer ; others live for two years, in the first accumu- 

 lating a store of food, in the second expending it in lavish 

 blossoming ; yet others live for many years. The annual 

 habit limits growth to modest dimensions, but enables the 

 plant to pass through seasonal extremes in the best of all 

 protected stages, as a seed ; it means, too, the possibility 

 of very rapid distribution. The dangers typically avoided 

 by the annual habit are those of drought. We have noted 

 the presence of short-lived grasses, the only shallow-rooted 

 plants, in the American prairies. In desert conditions 

 annuals are very numerous. On Tumamoc Hill, in the 

 Arizona Desert, Cannon (191 1) found 223 annual species, a 



