32 THE PLANT LIFE OF MARYLAND 



required by plants. The amplitude of the curve varies from place 

 to place, and the time during which it is below the ecological optimum 

 for the majority of plants increases as we pass from the Tropic of 

 Capricorn to the Arctic Circle, in other words the growing season 

 decreases in length as we pass northward in the North Temperate 

 Zone. The manner in which the length of the growing season affects 

 plants is different, in different species. In sub-tropical Florida 

 perennial plants maintain continued activity and herbaceous plants 

 spring up and come to maturity without relation to the time of the 

 year. Further north, in the latitude of Maryland, some of the 

 perennial plants, as our deciduous forest trees, become adjusted to 

 the winter season through leaf-fall, while other perennial plants, as 

 our pines and cedars, are permanently adjusted through the form 

 and structure of their leaves to the unfavorable conditions of winter. 

 The annual plants in tins latitude are brought into unison so as to all 

 germinate, grow and set seed during approximately the same months 

 of the year. Still farther north, near to the Arctic Circle, the short- 

 ness of the growing season gives the evergreen conifers an important 

 place numerically among the woody perennials, while the herbaceous 

 plants are either adjusted to the length of the season by rapidity of 

 development, or capable of withstanding arrest of development 

 through a freezing temperature, and of continuing their development 

 without backset on the return of spring. 



It is important to bear in mind that it is not the coldness of the 

 winter which determines directly the habit of leaf-fall in deciduous 

 trees, but that this phenomenon, which is equally observable in the 

 trees of the tropics in different months of the year, is brought into 

 unison in the temperate zones and prolonged through the existence in 

 winter of conditions unfavorable to the absorption of soil-water by 

 roots, — namely the frozen condition of the soil or its low tempera- 

 ture. In the coniferous evergreens the balance between absorption 

 and transpiration is maintained during the winter through the form 

 and structure of the leaves, not, however, without a consequent re- 

 striction of their capacity for activity during the favorable summer 

 season. In the perennials of the temperate zone the low temperatures 

 of winter are not fatal and tin- features of structure sometime- sup 

 posed to be protective against cold, — hairy or viscid winter buds, 



