YOUTH AND ADULT LEAVES 445 



or quite entire ; between the two extremes a series of 

 gradations may be traced. On a single shoot, in perennial 

 plants which form buds, the succession of foliage leaves is 

 interrupted by the bud scales, which represent leaves or 

 particular parts of leaves modified by the conditions of their 

 development, and related in structure to their function of 

 protection. Between these bud scales and the foliage 

 leaves transition forms may be traced in many cases, as in 

 the cherry and the horse chestnut. Similarly scale leaves 

 and transition forms are found at the base of the shoots of 

 many perennials. Very frequently the phyllotaxy of the 

 juvenile leaves differs from that on the mature regions of the 

 shoot ; in the sunflower the first leaves are opposite and 

 later a two-fifths arrangement is assumed. 



One aspect of the change from youth to adult form has 

 already been dealt with in the sun and shade leaves of trees. 

 Examples of more striking diff"erences are the harebell with 

 its round youth leaves and linear adult leaves ; the whin, 

 in which the first leaves are trifoliate and the adult leaves 

 spines ; many Acacias, which first produce a few pinnate 

 leaves and then phyllodes, sometimes with intermediate 

 forms. 



Of great interest is the condition found in various 

 conifers. In the pines the seedling stem bears single, spirally 

 arranged needles ; later the long shoots bear only numerous 

 membranous scales, and the needles are borne in pairs, or 

 larger numbers, on special short shoots. The youth needle 

 is longer and softer than the adult, and there are structural 

 differences. In Pinus syhestris the youth needles disappear 

 in the second year ; in other species, e.g. P. canariensis, 

 they may be produced for several years. In Chamaecyparis 

 and Thuja the seedling has needles, and the mature shoot 

 bears the characteristic cupressoid scales. The youth form 

 in this case, and in the similar case of Thuja, may be 

 fixed by using side branches as cuttings, and such plants 

 retain the youth form indefinitely. The fixed youth form 

 of Chamaecyparis is, in fact, cultivated in gardens under 

 a separate generic name — Retinispora. A Chamaecyparis 



