THIRD TYPE OF CONSTRUCTION 



287 



one another. This feature is partly connected with the necessity for 

 the presence of air-spaces in immediate contact with the photosynthetic 

 cells ; it also indicates that each palisade element has a tendency to 

 become independent of its neighbours, neither receiving raw food- 

 materials from the latter nor supplying them with synthetic products. 

 Such a palisade-cell in fact only carries on interchange of materials 

 with the tissues that are in contact with its two extremities, that is to 

 say, with the epidermis or the water-tissue on the one hand, and with 

 the spongy parenchyma or the parenchymatous bundle-sheaths on the 

 other. There is another circumstance which proves that no transloca- 

 tion takes place from one palisade-cell to another. At the base of the 

 leaf, namely, the development of the palisade-tissue is in no way 

 different from that which prevails at the distal end, although the 

 quantities of translocated material passing through 

 the cross-section of the leaf are very different in the 

 two cases. 



The stream of synthetic products in the pali- 

 sade-cells thus undoubtedly seems to follow the long 

 axis of the elements in ques tion, or, in other w ords, 

 to travel at right angles to the surface of the organ. 

 This interpretation also accounts for the existence 

 of a particular variety of palisade-tissue which is 

 otherwise inexplicable. Not infrequently a small 

 group of from two to ten palisade- cells con verge at 

 their lower ends so as to form a little farTshaped 

 group resting upon a single un derlying cell, th e 

 upper end of which is correspondingly dilat ed in a 

 funnel-shaped manner (Figs. 1 16 and 117); t he 

 obvious inference is that these supporting elements 

 are collecting-cells, which receive the synthetic products from all the 

 members of a group of palisade-ce lls, and t fa n srrrit ~th e m more or less 

 directly to the main chan nels of transl ocation. The author has noticed 

 this peculiar feature of the photosynthetic system in Ficus elastica 

 where it is particularly wll -d^vjippd fnrthr i n Pulmem a ria, njftf.imyl i r r, 

 Juglans regia, Eleagnus angustifolia, Eranthis hiemalis, etc. Sometimes 

 the groups of palisade -cells abut directly upon elements of the inter- 

 mediary tissue, especially when the leaf is thin and not very highly 

 differentiated. 



The intermediary tissue, which constitutes the physiological link 

 connecting the photosynthetic tissue with the efferent channels, is 

 represented by the familiar spongy parenchyma. It lies beneath the 

 nalisade-layers, and generally consists of elements provided with a 



$-7 greeted branches, constituting so many 



Fig. 117. 



Small portion of a 

 T.S. through the leaf 

 of Ficus elastica, show- 

 ing a collecting-cell 

 and its associated 

 group of palisade-cells. 



