32 BULLETIN 680; U. S. DEPAETMENT OF AGKICULTTJRE. 



sunlight produce the largest amount of seed. Even small scrubby 

 trees in open places usually bear relatively large crops of seed. The 

 seed has a comparatively low rate of germination (15 to 45 per cent) 

 and fairly persistent vitality. Because of their large light wings 

 the seeds are widely distributed by wind. Germination of the seed 

 and growth of seedlings are best in moist vegetable mold, but excel- 

 lent also on moist mineral soil. Partial shade is favorable to early 

 growth of seedlings because soil moisture is conserved under such 

 conditions. Lack of sufficient soil moisture during the critical early 

 period of growth often results in the death of large numbers of seed- 

 lings. Good reproduction occurs under the partial shade of old 

 trees, but it is particularly abundant in open places in thin stands, 

 where dense thickets of young incense cedar frequently exclude other 

 seedling trees. In cool moist places, however, white fir may enter 

 such thickets and later become predominant. Incense cedar is often 

 the first conifer to establish itself at the lower edge of its vertical 

 range in chaparral and oak growth, proving its great adaptation to 

 different degrees of light, moisture, and character of soil, as well as 

 its general hardiness during seedling stages of growth. 



LONGEVITY. 



Much is yet to be learned regarding the age limits of this tree. 

 So far as is now known, it is long-lived, but the ages of very large 

 trunks have not been determined, chiefly because the centers of old 

 trees are often so completely destroyed by fungi as to prevent count- 

 ing the earlier growth rings. Narrow-ringed trees from 24 to 36 

 inches in diameter are from 360 to 546 years old. One wide-ringed 

 tree examined by the author was 42 inches in diameter and only 

 115 years old, showing exceptionally rapid growth. The largest trees 

 are probably from 600 to 800 years old, or possibly one or two hun- 

 dred years older. 



GENERIC CHARACTERISTICS OF ARBORVITvES. 



The trees belonging to the group which is technically called Thuja 

 are commonly known as ''cedars" or arborvitse.^* They are medium 

 to very large-sized evergreen trees. The adult form of foliage con- 

 sists of minute, overlapping, scalelike leaves, arranged as in Libo- 

 cedrus, and conspicuously flattened on the short side branchlets, 

 which are arranged in one plane, thus forming flat sprays (PI. XI, 

 XII). The back of the leaf is marked with a tiny gland or small pit- 

 like depression. Seedling leaves are narrowly lance-shaped and 

 sharp-pointed, and spread widely from the stem (PI. XI, 6). 



** Nurserymen and many writers treating of coniferous trees from the standpoint of horticulture appear 

 to favor calling species of the genus Thuja arborvitfES. A number of botanical writers also favor this. 

 Lumbermen, woodsmen, and others concerned with the use of the wood of these trees, however, uniformJy 

 or with few exceptions, designate them as cedars. 



