202 ALTERATION OF POSITION. 
stage of development which it has 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 Sechiwm 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 stance. 
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 Babylonica, 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 Itahan poplar. 
M. de Selys-Longchamps has described a similar 
occurrence in another species of Poplar (P. virginiana 
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. 
