MORPHOLOGY 47 



Junipers, that now have scale-like compressed leaves, have been derived 

 from Conifers with needle-shaped leaves. This conclusion is further 

 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 young 

 plant has attained a certain age, while in some Junipers needle-shaped 

 leaves are retained throughout their whole existence. Even still more 

 instructive are the Australian Acacias, whose leaf - stalks become 

 modified, as phyllodia (p. 35), to perform the functions of the reduced 

 leaf- blades. The proof for such an assertion is furnished by a 

 germinating plantlet of Acacia pycnantha (Fig. 48), 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 

 perpendicular direction. At length, leaves are produced which 

 possess only broad, flattened leaf-stalks. As many other species of 

 this genus are provided 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. 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. 



II. INTERNAL MORPHOLOGY 



(Histology and Anatomy) 



The Cell 



All plants, as all animals, are composed of elementary organs 

 called cells. In contrast to animal cells, typical vegetable cells are 

 surrounded by firm walls, and are thus sharply marked off from 

 one another. In fact, it was due to the investigation of the cell walls 

 that the cell was first recognised in plants. An English micrographer, 

 Eobert Hooke, was the first to notice vegetable cells. He gave them 

 this name in his Mkrographia in the year 1667, because of their 

 resemblance to the cells of a honeycomb, and published an illustration 

 of a piece of bottle-cork having the appearance shown in the adjoining 

 figure (Fig. 49). Eobert Hooke, however, was only desirous of ex- 



