Stems are sometimes developed into long runners or stolons.  These circumnutate in a conspicuous manner, and are thus aided in passing between and over surrounding obstacles.  But whether the circumnutating movement has been increased for this special purpose is doubtful. [page 559]

We have now to consider circumnutation in a modified form, as the source of several great classes of movement.  The modification may be determined by innate causes, or by external agencies.  Under the first head we see leaves which, when first unfolded, stand in a vertical position, and gradually bend downwards as they grow older.  We see flower-peduncles bending down after the flower has withered, and others rising up; or again, stems with their tips at first bowed downwards, so as to be hooked, afterwards straightening themselves; and many other such cases.  These changes of position, which are due to epinasty or hyponasty, occur at certain periods of the life of the plant, and are independent of any external agency.  They are effected not by a continuous upward or downward movement, but by a succession of small ellipses, or by zigzag lines,—­that is, by a circumnutating movement which is preponderant in some one direction.

Again, climbing plants whilst young circumnutate in the ordinary manner, but as soon as the stem has grown to a certain height, which is different for different species, it elongates rapidly, and now the amplitude of the circumnutating movement is immensely increased, evidently to favour the stem catching hold of a support.  The stem also circumnutates rather more equally to all sides than in the case of non-climbing plants.  This is conspicuously the case with those tendrils which consist of modified leaves, as these sweep wide circles; whilst ordinary leaves usually circumnutate nearly in the same vertical plane.  Flower-peduncles when converted into tendrils have their circumnutating movement in like manner greatly increased.

We now come to our second group of circumnu-[page 560] tating movements—­those modified through external agencies.  The so-called sleep or nyctitropic movements of leaves are determined by the daily alternations of light and darkness.  It is not the darkness which excites them to move, but the difference in the amount of light which they receive during the day and night; for with several species, if the leaves have not been brightly illuminated during the day, they do not sleep at night.  They inherit, however, some tendency to move at the proper periods, independently of any change in the amount of light.  The movements are in some cases extraordinarily complex, but as a full summary has been given in the chapter devoted to this subject, we will here say but little on this head.  Leaves and cotyledons assume their nocturnal position by two means, by the aid of pulvini and without such aid.  In the former case the movement continues as long as the leaf or cotyledon remains in full health; whilst in the latter case it continues only whilst the part is growing.  Cotyledons appear to sleep in a larger proportional number of species than do leaves.  In some species, the leaves sleep and not the cotyledons; in others, the cotyledons and not the leaves; or both may sleep, and yet assume widely different positions at night.