120 ROBERT TRACY JACKSON ON 



Two specimens of seedling Arbor Vitae, growing under a hedge in cultivation at 

 Andover, Mass., were collected by Mi's. R. T. Jackson. In both specimens, the first 

 leaves succeedhig the cotyledons are a pair of opposite leaves. In one of the specimens 

 (PI. 22, fig. 86) , the second whorl is four-leaved, and succeeding leaves ai'e in whorls of 

 threes on the main stem; but a lateral branch has leaves in twos only. The second 

 specimen has three leaves in the second whorl ; but it is evidently a four-leaved whorl in 

 which one leaf is suppressed, or lost in early growth, as a space exists where it should 

 occur. Later leaves of this specimen are arranged on the main stem in irregularly placed 

 threes, they not being sufficiently in the same planes of insertion to call them whorls. 

 On a side branch the leaves are in twos as usual. In a seedling figured in Professor 

 Sargent's Silva, there are two cotyledons succeeded by a whorl of two leaves, then by 

 several whorls of four leaves. While considerable variation occurs, it is evident that the 

 typical seedling has two cotyledons succeeded by a whorl of two leaves, then by one or 

 more whorls of four leaves succeeded by the two-leaved whorls characteristic of this 

 type. The variations are no greater than those described later in Lihocedrus decurrens. 



To return to the specimen of Arbor Vitae (PI. 22, fig. 84), the narrow, elongate, 

 primary leaves are maintained to the tip of the main stem, and the same feature exists 

 in all the other seedlings; concurrently lateral branches have already acquired leaves 

 which are a close approach to the adult condition. This is a notewoi'thy feature, 

 for in many conifers, Pinus rirjida, Fig. 74, Libocedrus, Fig. 89, the main stem develops 

 mature features first, or at least as soon as lateral branches. On the side branches (PI. 

 22, fig. 84) , at their inception, there are one or more pairs of typical primary leaves ; but 

 these soon give place to leaves which are shorter, appressed, scale-like. Succeeding 

 pairs of leaves (PI. 22, fig. 84b) diflier, as do adults, in that the marginal leaves are narrow, 

 embracing by their apposed edges the median pairs which are flattened, broadened and 

 embraced by the distal borders of the preceding marginal pair. These leaves of young 

 branches are acuminate distally, with free tips, whereas in the adult (PI. 22, fig. 85) the 

 tips of the leaves are blunt and appressed. It is an interesting phylogenetic fact, that in 

 fossil Thuyites from the Jurassic, the tips of the leaves are acuminate and distally free as 

 in the young of our species (Fig. 84b) . 



The features as described are characteristic of 35 out of 40 seedlings of this species 

 from the Adirondacks; 5 specimens only showed a variation. In three of these the leaves 

 are in whorls of threes up to within two or three whorls of the introduction of " charac- 

 ter " leaves, when two-leaved whorls appear. Two other specimens have respectively 

 14 and 17 whorls of three leaves each, above which the leaves are in twos as usual. It is 

 to be remembered, as stated, that only one specimen showed cotyledons and first suc- 

 ceeding whorls. This taking on of a three-leaved whorl in a type which is so strongly 



