388 A TEXTBOOK OP BOTANY [Ch. VIII, 6 



rangement as the leaves, tend to build stem-and-leaf structures 

 very symmetrical in plan. Meantime also the special tissues 

 which give strength and meet other needs are continuing to 

 develop in places required by stress or other demand. 



In this stage appears the striking seasonal cycle imposed 

 on all plants outside of the tropics by the extreme alter- 

 nation between summer and winter. The summer alone has 

 the warmth to permit full vital activity in plants, and ac- 

 cordingly is the season of green vegetation, accumulation of 

 food, and development of new parts. In the autumn prep- 

 aration is made for the wdnter, and accordingly that is the 

 season when fruits are ripened, buds are enwTapped in their 

 scales, leaves are cut off and dropped, and tissues are par- 

 tially dried ; while the attractive colors of fruits and the 

 varied hues of dj-ing leaves make it a time of Ijright color 

 in vegetation. The winter is the season of enforced dor- 

 mance, when the dried tissues of plants, approaching the 

 conditions in seeds, remain almost inactive within their 

 nearly sealed wrappings, which display no colors other than 

 their incidental grays or bro^^^^s. The spring is the season 

 of unfolding, when the ready-formed parts, amply supplied 

 with stored food, absorb copious water, enlarge, burst their 

 wrappings, and push forth green leaves to make new food, 

 and bright flowers to effect fertilization ; and all vegetation 

 wears the soft colors of the new-forming tissues. This is 

 the four-part seasonal cycle through which our perennial 

 plants pass every year as long as they live. 



The next stage of the developmental cj'cle is the adult. 

 It is not distinguished from the sapling by attainment of an_v 

 fixed size, for plants (unlike animals) continue to grow, by 

 formation of new parts, as long as they live. Nor is it 

 marked by any change in the mode of formation of roots, 

 buds, or leaves, which continue to be made in the same gen- 

 eral way. It is true, a gradual loss of the youthful sym- 

 metry accompanies advancing age in trees and shrubs, 

 partly because of the interference of the over-plentiful 



