THE LORANTHACE^ OF CE^LO^^ 107 



these circumstances also, nutation curvatures, due to tlie irregular growth of the 

 hypocotyl, occur. 



If reference be made to woodcut 6, it will be seen that the nutation which occurs in 

 these hypocotyls may in some degree be likened to the "undulating "nutation described 

 by Wiesner * and others in the case of growing •* epicotyls," whereby the whole epicotyl 

 assumes an S-shaped curvature. If the series of figures showing the development of 

 Lorantluis loniceroides under ordinary circumstances be examined (fig. 6, PI. XI.), it will 

 be seen that what indeed is normally present is exactly such an S-shaped curvature ; 

 that at first, owing to the apparently innate nutation aided by the negative heliotropism, 

 the hypocotyl bends down tow^ard the shaded side ; but at a considerable time after the 

 suctorial disc (the head of the hypocotyl) has become fixed to its support, the upper end 

 of the hypocotyl (that nearest to the plumule) grows more on its concave side, and thus 

 efl^ects the straightening out of the whole hypocotyl, so that its axis is now a straight 

 line. This fact has been interpreted f to indicate a " somatropic " [)0wer on the part of 

 the hypocotyl ; but it may be doubted whether the term facilitates the understanding of 

 the process. 



This nutation which occurs in Loranthm independent of light and of gravity has been 

 dwelt on at some length, partly because of the general bearing the facts have on the 

 theory of nutation, but chiefly because the view generally held that the curvature of the 

 hvpocotyl of Lorantlms (or Viscum) is merely an expression of the directive influence of 

 light (negative heliotropism) must, I think, be modified thus : — the ordinary curvature 

 of the hypocotyls of such Loranths as L. loniceroides is not merely due to negative 

 heliotropism; but is the resultant of this, together with nutation. But that this 

 nutation is in itself highly important for the success of the parasite in any anomalous 

 position in which it may find itself has already been practically illustrated by the 

 account given of the L. necUjherrensis seedling which, though grown in the dark, 

 succeeded by its nutation in bringing its " suctorial disc " to the host. 



It not unfrequently happens that seeds of L. loniceroides, having been placed or 

 dropped on a branch, topple over so that they come to lie upon the branch. The 

 hypocotyl of such a seed grows out and bends immediately by negative heliotropism 

 towards the branch (cf. woodcut 4 a), and its head, reaching this branch before it is more 

 than 3 or i mm. in length, fails to remain there, being carried away by the elongating 

 and nutating hypocotyl. A similar condition of aff'airs is brought about when the seed 

 of L. loniceroides is so placed that its suctorial disc must soon come in contact with the 

 branch [cf, woodcut 4 b). 



The " force " of this nutation is sufficient to overcome the power of light combined 

 with any influence which contact (see later) has on its *' suctorial disc." This is 

 exemplified by the behaviour of a hypocotyl of a developing L. loniceroides seed, so 

 placed that the hypocotyl will grow downward on to the branch which is 8 or 4 mm. 

 below it. Under these circumstances, as in the last case, the head of the hypocotyl, 



* Sitzungsber. d. k, Akad. in Wion. Ixxvii. (1878). 

 t Vines, ' Physiology of Plaats,' p. 47*i. 

 SECOND SERIES. — BOTANY, VOL. V. Q 



