108 ME. F. W. KEEBLE ON 



after reaching the branch, and even pressing against it, is carried away by the nutation 

 of the growing hypocotyL 



To briefly sum up :— Though it is perfectly true that the hypocotyls of such members 

 of the genus Lorantlius as L. loniceroides are, as has already been proved by others to 

 be the case with Viscum album, negatively heliotropic and ageotropic, yet an important 

 means in addition to the negative heliotropism, w^hereby the curvature of the hypocotyl 

 is brought about, is the nutation w^hicli occurs in these hypocotyls. By this nutation 

 tlie " suctorial disc " may be brought, from the position into which negative heliotropism 

 brings it, to point directly toward the light. If no branch or object of attachment is 

 available, the hypocotyl may ultimately become coiled up very like some tendril which 

 has not met with a support. By this nutation the " suctorial disc," if it fails to reach 

 the host by the direction which negative heliotropism impresses upon it, is brought into 

 a series of positions whereby its chances of obtaining some hold on a suitable object are 

 enhanced. 



It may be remarked, in conclusion, that these hypocotyls are apparently possessed of 

 more bilaterality of structure than those of ordinary seedlings, and, indeed, it is by no 

 means uncommon to find the axis of the embryo obviously curved. Now Sachs has long 

 ago pointed out " that seedlings of Dicotyledons are remarkable illustrations of bilateral 

 structures which nutate in one plane " *, so that it may be that the more irregular 

 nutation which occurs in the Loranths, above mentioned, is associated with tbe greater 

 structural irregularity of their hypocotyls. 



G^^oicfh of Hypocotyl. — On the germination of the embryo of L. loniceroides , the head 

 of its hypocotyl is pushed out from the endosperm by growth in a region just behind 

 this head ; later growth is greatest in the most recently emerged zone. 



At a later stage, when the hypocotyl has reached a length of about 1 cm., an inter- 

 calary region occurs in which the rate of growth is higher than in zones on either of its 

 sides. A third and smallest maximum of growth connected with the increase of the 

 sucker occurs at the distal end of this latter organ. 



The grow^th of the head (suctorial disc) is, as will be shown in the anatomical section 

 of the paper w4iich is to follow this, connected with the later development of the main 

 (penetrating) root, a development which proceeds to some extent when the head is 

 growing free and not in contact either with a branch or even with an inert body. This 

 growth is especially marked in L. neelgherrensis, and has been mentioned in the 

 description of the germination of this species. 



To these observations on the growth of the sucker may be added some on the enlarge- 

 ment of the head (suctorial disc). 



Under ordinary circumstances the head of the hypocotyl of L. loniceroides, on reaching 

 the host, swells considerably, becoming conical (fig. 6, PI. XI.), with its broad base 

 firmly afiixed to the branch. When, however, the seedling is so placed that the head of 

 the hypocotyl is free, this swelling does not take place ; but the originally flat surface 

 of the suctorial disc becomes first regularly convex, and then a number of irregular 

 projections appear on its surface. 



* Sachs, ' Text-book of Botany,' p. 857 ; English translation, 2nd edition. 



