202 ALTERATION OF POSITION. 



stage of development wliicli it lias arrived at, to unequal 

 or disproportionate growth of some parts, or to the 

 presence of some impediment either accidental or result- 

 ing from the natural growth of the plant. These and 

 other causes tend to alter the direction of parts very 

 materially. 



Change in the direction of axile organs, roots, stems, &c. 

 The roots frequently exhibit good illustrations of the 

 effect of the causes above mentioned in altering the 

 natural direction. The roots are put out of their 

 course by meeting with any obstacle in their way. 

 Almost the only exception to the rule in accordance 

 with which roots descend under natural circumstances, 

 is that furnished by Trapa natans, the roots of which 

 in germination are directed upwards towards the sur- 

 face of the water. So in Sechium edule, the seed 

 of which germinates while still in the fruit, the roots 

 are necessarily, owing to the inverted position of the 

 embryo, directed upwards in the first instance. 



A downward direction of the stem or branches 

 occurs in many weak-stemmed plants growing upon 

 rocks or walls, or in trees with very long slender 

 branches as in Salix Babylon ica^ and the condition may 

 often be produced artificially as in the weeping ash. 



The opposite change occurs in what are termed 

 fastigiate varieties, where the branches, in place of 

 assuming more or less of a horizontal direction, become 

 erect and nearly parallel with the main stem as in the 

 Lombardy poplar, which is supposed to be merely a 

 form of the black Italian poplar. 



M. de Selys-Longchamps has described a similar 

 occurrence in another species of Poplar (P. mrginiana 

 Desf.), and amongst a number of seedling plants 

 fastigiate varieties may frequently be found, which 

 may be perpetuated by cuttings or grafts, or some- 

 times even by seed ; hence the origin of fastigiate 

 varieties of elms, oaks, thorns, chesnuts, and other 

 plants which may be met with in the nurseries. 



