54 
fore, we should expect growing plants, with slender mobile 
stems, to bend towards the light. And such is, in fact, the 
case. I quote from Darwin : The Movements of Plants, 
page 465: — ''In our various experiments we were «nften 
struck with the accuracy with which seedlings pointed to a 
light, although of small size. To test this, many seedlings of 
Ph alar is, which had germinated in darkness in a very nar- 
row box several feet in lengbli, were placed in a darkened 
room near to and in front of a lamp bearing a small cylindri- 
cal wick. The cotyledons at the two ends and in the central 
part of the box would, therefore, have to bend in widely 
different directions in order to point to the light. After they 
had become rectangularly bent, a long white thread was 
stretched by two persons, close over and parallel, first to one 
and then to another cotyledon ; and the thread was found in 
almost every case actually to intersect the small circular wick 
of the now extinguished lamp. The deviation from accuracy 
never exceeded, as far as we could judge, a degree or two." 
Of course, in such cases, it may be objected that chloro- 
phyll is not yet fully formed ; but, inasmuch as chlorophyll is 
very quickly developed in the light, it may be supposed that 
the process of its formation, and the consequent accelerated 
synthesis of proteid, begins at once ; while plenty of time was 
allowed for the reaction, since, in the experiment just before 
the one quoted, eight hours was allowed for seedlings of 
Brassica and Phalaris to bend ''rectangularly towards the 
light." 
In order to see how intimately the bendin"- of plants 
towards the li^ht depends upon the illumination of the 
chlorophyll, it is only necessary to refer to Darwin's "Move- 
ments of Plants," page 449 to page 468. 
The few exceptions nearly all admit of some other expla- 
nation. Thus, Darwin shows that heliotropism may be much 
modified in some plants owing to their habit of climbing : in 
other cases apheliotropism may be induced because too intense 
illumination injures the chlorophyll,'^" and therefore reverses 
the effect we have described. Further, in time, the prepon- 
derating growth of the illuminated side will tend to reverse 
the effect. In the rare cases where plants containing little or 
no chlorophyll are heliotropic we may assume that light aids 
assimilation in some other way. The tendency for leaves lo 
place themselves perpendicular to any not too strong illumi- 
nationf is easily understood when we consider the influence 
of illumination upon the leaf stalk ; illumination of its upper 
surface will cause a diminution of that surface — as we have 
* Dar^-in : The Movements of Plants, page 446. 
t Ihid., page 449. 
