MORPHOLOGY 163 



of the development of a plant from the embryo is also, as a rule, 

 applicable to its further growth from the growing point, and, conse- 

 quently, a knowledge of the mode of development at the growing 

 point is of great importance in establishing homologies. The earlier 

 a characteristic makes itself apparent in the embryo, or the nearer it 

 is to the growing point of the old plant, so much the greater is its 

 value in determining the general relationships existing between the 

 different plants ; the later it is exhibited in the embryo, or the 

 farther removed it is from the growing point of the plant, the less 

 its general value, but the greater, in proportion, its importance in 

 defining the character of a genus or species. From the fossil remains 

 of former geological periods, it is safe to conclude that such Conifers 

 as Thuja, Biota, and the various Junipers, that now have scale-like 

 compressed leaves, have been derived from Conifers with needle- 

 shaped leaves. This conclusion is confirmed by the fact, that on the 

 young plants of the scaly-leaved Conifers typical needle-shaped leaves 

 are at first developed. The modified leaf forms do not make their 

 appearance until the so-called JUVENILE FORM has attained a certain 

 age, while in some Junipers needle-shaped leaves are retained through- 

 out their whole existence ( 158 ). Even still more instructive are the 

 Australian Acacias, whose leaf-stalks become modified, as phyllodia 

 (p. 45), to perform the functions of the reduced leaf-blades. The 

 demonstration of such an assertion is furnished by a germinating 

 plant of Acacia pycnantha (Fig. 171), in which the first leaves are 

 simply pinnate, and the succeeding leaves bipinnate. In the next 

 leaves, although still compound, the leaf-blades are noticeably reduced, 

 while the leaf-stalks have become somewhat expanded in a vertical 

 direction. At length, leaves are produced which possess only broad, 

 flattened leaf-stalks. As many other species of this genus are pro- 

 vided only with bipinnate leaves, it is permissible on such phylogenetic 

 grounds to conclude that the Australian Acacias have lost their leaf- 

 blades in comparatively recent times, and have, in their stead, 

 developed the much more resistant phyllodes as being better adapted 

 to withstand the Australian climate. The appearance, accordingly, 

 of the phyllodes at so late a stage in the ontogenetic development of 

 these Acacias is in conformity with their recent origin. Lathyrus 

 aphaca (p. 44) the leaves of which in the mature plant are transformed 

 into tendrils, has the first leaves of the seedling provided with 

 leaflets. It may, in like manner, be shown that in the case of 

 plants with similarly modified leaf forms, the metamorphosis of the 

 leaves does not take place until after the cotyledons and the first 

 foliage leaves have been developed, and it is then usually effected 

 by degrees. 



In the lower divisions of the vegetable kingdom, especially among 

 the Fungi, PLEOMORPHISM is a phenomenon of frequent occurrence. 

 In the developmental cycle of the species more than one living being 



