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ANATOMY, AND LIFE-HISTORY OF THE CONIFERS. 247 



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homotaxic, but much longer than ordinary, and owing to the 

 rarity with which lateral buds are formed the main branches are 

 elongated and unbranched, suggestive of a resemblance to snakes. 

 It is interesting to compare these forms with the arrangement 

 in young plants of Araucaria imbricata. These curious firs will 

 be spoken of again under the head of " ramification." 



Changes in the appearance and disposition of the leaves also 

 occur as a result of alternations of growth and of arrest of growth 

 in various degrees. Thus, when growth first begins in spring, 

 it proceeds rapidly, and then may come a period of arrest and con- 

 tracted growth. Thus we may have, at different parts of the same 

 branch, long leaves arranged in a pseudo-distichous manner, 

 and others much shorter and spreading on all sides, as shown in 

 the accompanying illustration from Abies Nordmanniana (fig. 6). 



The genus Podocarpus presents marked illustrations of hetero- 



morphy and of heterotaxy in the foliage. The leaves on some of 



the quickly growing shoots are distichous, arranged in one flat, 



horizontal plane, the individual leaves being broadly linear and 



somewhat falcate, while on the older or less quickly growing 



shoots the leaves are polystichous, short, subulate, and appressed 

 to the stem. 



Another instance of variation in the arrangement of leaves is 

 often seen in Abies Nordmanniana, A. Pichta, A. amabilis, as 

 also in Tsuga canadensis, &c. The leaves on the lateral and 

 more or less horizontally spreading branches, though polystichous, 

 in reality arrange themselves in three rows, one on either side of 

 the branch (in which case the leaves are nearly at a right 

 angle to the branch), and one in the median plane of the upper 

 surface (ia which case the leaves are appressed along the branch 

 parallel to its main axis). The median leaves are usually smaller 

 than the lateral ones. This arrangement may be compared to 

 that in the species of Selaginella, or to that in Lycopodium 

 spectabile, L. volubile, &c7, where the lateral leaves are relatively 

 iar ge and decurrent, while the median leaves (dorsal as well as 

 ventral iu these instances) are relatively small, free at the base 

 and caducous. In Abies grandis the uppermost row of leaves is 

 not arranged parallel to the axis, as in the instances just referred 

 to, but at right angles in the ordinary way, the obstruction caused 

 b Y the overlapping leaves being minimized by the smaller size of 



? upper leaves, and by the power which the leaves have of 









Rising or depressing themselves according to circumstances. 











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