12 JOUKNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



notes that other stolons on the ground, as strawberry runners, have a strong 

 tendency to "congregate in more than usually shaded places."* Now 

 it must be emphasized that many habits of plants due to growth 

 originally caused by responses to external stimuli may become so per- 

 manently fixed in the life of the species that they occur when the original 

 stimulus is not present. 



This permanency of acquired habits is thus well described by Professor 

 Vines. In speaking of the periodicity in the circulation of water in 

 plants, he says : " It has doubtless been induced in plants by the daily 

 variations of external conditions, perhaps more especially of illumination, 

 which are involved in the alternation of day and night ; but it has become 

 so much a part of the nature of plants, that it is exhibited, even when the 

 conditions which originally induced it are not present, and it is transmitted 

 from generation to generation, "t Darwin corroborates such tendencies 

 to become hereditary. + 



There is reason to think that an acquisition of any special habit may 

 become fixed and hereditary. Perhaps one of the most conspicuous is 

 the orthotropism of the terminal part of a stem in total darkness when it 

 has been placed horizontal, already referred to. 



It is customary to attribute this orthotropism to gravitation ; but we 

 must remember that erect stems grow in direct opposition to it ; and, to 

 do this, the stem develops varying amounts of mechanical tissues, always 

 somewhat in excess, to resist the ever existing downward "pull" of 

 gravitation, and other strains due to wind, &c. 



A remarkable instance of change of direction is seen when the tap- 

 root or the terminal shoot of the stem is removed. In the former case, 

 the secondary previously horizontal roots grow downwards, and the 

 boughs near the top grow upwards. These changes must be partially 

 at least attributable to some sudden impulse due to the checks given to 

 the flow of sap in both cases, coupled with gravitation and light. 



A permanent result of the former is seen in all Monocotyledons and 

 aquatic Dicotyledons, as in neither is the tap-root preserved, but numerous 

 adventitious roots arise from the stem and grow downwards, supporting 

 it as well as absorbing nourishment. Analogous instances may be seen 

 in the two varieties of trees called "fastigiate" and "weeping." In the 

 former all the branches grow orthotropically, and in the latter in a pendulous 

 manner. How these habits arise is not known. It appears they may 

 (as in Irish yew), or more probably may not, be hereditary, as in the ash. 

 Seedlings sometimes show at first a slight tendency "to weep," but 

 subsequently lose it. 



Sachs gives an excellent illustration to show these inherited tendencies 

 to orthotropism. A Yucca gloriosa growing in a pot, with its vertical 

 rhizome, is inverted. In this condition buds on the rhizome, as well as 

 roots are developed from it, the former grow vertically upwards and the 

 latter downwards, though both are, of course, in total darkness, as they 

 are buried in the soil. § 



* Gardeners' Chronicle. 



f Physiology of Plants, p. 96. 



% Movements of Plants, pp. 407, 491. 



§ Sachs' Physiology of Plants, p. 529, fig. 344. 



