MOVEMENTS OF LEAVES. 



29 



leaves are distichous, that is to say, they are arranged in opposite rows on 

 either side of the shoot, and spread in one plane. In the common Yew, 

 Torreya and Cephalotaxus the leaves are sub-spirally arranged around the 

 shoot but spread laterally in two directions only, except in the fastigiate 

 forms already noticed. In Abies, Picea, (with some exceptions,) 

 Tsuga and Abietia, the leaves on the under side of the lateral shoots are 

 twisted at their base so as to bring them either into a horizontal position 

 or at a small angle to it, arranged in two, three or more ranks ; whilst 

 those on the upper side are nearly parallel with the long axis of the shoot, 

 more or less appressed to it, and are always shorter than those pointing 

 laterally,* as in Abies amabilis, A. Nordmanniana, Picea excelsa, P. nigra, 

 P. ajanensis, etc. ; but in Abies yrandis, A. balsamea and A. concolor all 

 the leaves of the sterile branches are pseudo-distichous. It is now well 

 known that these positions are taken by the leaves in order to bring 

 them into the most advantageous relation to direct sunlight, and thence 

 for the object of promoting their main functions to the greatest benefit 

 of the tree, viz., the assimilation of food stuffs, respiration, and the 

 exhalation of superfluous water vapour. 



In connection with the same objects may be mentioned the movements 

 of the leaves of some Conifers that have been observed during the 



their cicatrices. 



A. Ahics pectinata. B, Tsiiga canadensis. c, Abietia Douglasii. D, Picea excelsa.. 

 Libani. F, Larix europcea. G, Laricopsis Kcempferi. 



(From Engler and Prantl's Naturlichen Pflanzenfamilien). 



K, Ci'dnts 



season of active vegetation, as Abies Nordmanniana, Pinus halepensis 

 and other Pines, notably those with long filiform leaves as P. excelsa, 

 P. Strobus, etc. The leaves of these Pines in winter collapse and 

 hang in clusters that are quite pendulous; but in summer, especially 

 under the influence of sunlight, they become divergent and even spread- 

 ing. More observations covering a larger range of subjects are, however, 

 wanting before any certain deduction can be drawn from these 

 movements. 



In many of the species included in the Cypress tribe, especially those 

 in which the branchlets are frond-like, the decussate pairs of scale-like 

 leaves are frequently dissimilar, markedly so in Libocedrus, the lateral 

 pairs being larger and free at the apex, whilst the dorsi-ventral pairs, 

 those on the upper and under side of the shoot, are smaller and 



* Among the Abies, A. Pinsapo is a constant exception to this rule, and A. cephalonica 

 in part ; in these species the leaves spread equally in all directions on the branches, and on 

 their ramifications. Among the Piceas, P. polita and P. SmitMana are exceptions so far 

 as the trees growing in England have been observed. 



