350 Transactions 



noticeable in plants with creeping or underground stems. It is considered 

 probable that moisture is tbe chief factor determining their production, 

 and this view is supported by the positions of the roots in the lianes 

 observed. They are of great value in obtaining food for the lianes, and 

 by their efficiency in this respect they enable the plants to reproduce 

 vegetatively to a marked extent. The vigorously growing shoots which 

 may arise near these roots receive a sufficient supply of food from them, 

 and thus can soon reach the adult stage, so that in a short time any 

 portion of the forest may contain at least a dozen potential individuals 

 all derived from a single plant. 



A point of equal interest is the striking heterophylly of many of the 

 species. These plants possess a juvenile leaf differing in form to varying 

 extent from the adult leaf, and between the two forms there are all transi- 

 tional stages. Much attention has been directed to the phenomenon by 

 L. Cockayne, whose views in part seek to explain the different leaf-forms by 

 reference to past changes in the environment of the plants. Thus, after 

 dealing with Parsonsia, he writes, " This, taken in conjunction with the 

 fact that about 200 species of New Zealand plants- — -i.e., some 12 per cent, 

 of the spermophytes — belonging to most diverse genera and natural orders, 

 exhibit heterophylly of a more. or less striking character in their life-histories, 

 seems to distinctly point to there being some reason in New Zealand itself 

 for this special phenomenon ; and this reason, it seems to me, must be 

 sought for in the manifold changes which the geological history of the New 

 Zealand Archipelago has brought about " (1908, p. 488). And with regard 

 to the leaf-form of Parsonsia itself, " It seems clear that the possibilities 

 of both juvenile and adult are latent in the one plant, but each requires 

 its necessary stimulus to set it free in its entirety. If the stimulus is not 

 sufficient, then one or the other form may persist, or there may be a com- 

 bination of characters, as in the transitional forms " (1912, p. 24). 



In reviewing the features of the leaves of the lianes it is seen that there 

 are many characters common to them all. Also the shade leaves present 

 many differences from the sun leaves. 



In shade leaves the characters are as follow: (1.) The leaf-blades are 

 relatively expanded and membranous. In Rtihus cissoides, whose leaves 

 are reduced to midribs, these midribs are much longer than are those in 

 the open. (2.) Petioles are elongated. (3.) Cuticle is not well developed. 

 (4.) Lateral wails of epidermal cells are more wavy on the under leaf-surface. 

 (5.) Palisade cells are not closely packed. (6.) Spongy tissue is well 

 developed, and has irregular cells, between which are large intercellular 

 spaces. (7.) Stomata are on lower surface only. 



In sun leaves, on the other hand : (1.) Leaf-blades are smaller and more 

 coriaceous. (2.) Petioles are shorter. (3.) Lateral walls of epidermal cells 

 are straighter than in shade leaves. (4.) There is in most cases a thick 

 cuticle. (5.) Palisade tissue is well developed, with the cells elongated and 

 very closely packed. (6.) Spongy tissue comprises a smaller portion of the 

 thickness of the leaf, and has small intercellular spaces. (7.) Stomata are 

 on lower surface only, but more numerous than in shade leaves. 



The above features appear to present the following advantages :— 



The expansion of the leaf in the shade is advantageous in that a 

 larger number of light-rays will fall upon the assimilating surface ; and 

 the utilization of light is further aided by the broad palisade cells, which 

 give the chloroplasts a more superficial position than they occupy when 

 the cells are narrow and elongated. 



