HUTCHINSON'S POPULAR BOTANY 



Photo 6y] [E. Step 



FIG. 120. AN ALPINE LICHEN (Gyrophora cylindrica). 

 A dull purplish Lichen with beautifully fringed margins, that grows on Alpine rocks. Slightly enlarged. 



surface cells of short focus are connected with short palisade cells. The 

 whole subject, however, is in need of further investigation. 



In the leaves of Gymnospermous plants but not exclusively confined 

 to them is found a particular form of tissue, known as transfusion tissue, 

 which has been the subject of considerable controversy. In the leaves of 

 Conifers and most Cycads it is nearly always found in lateral connection 

 with the vascular bundles, in some genera outside the phloem, and in others 

 opposite the xylem. The highly developed network of conducting tissue so 

 prominent in the leaves of Dicotyledons is entirely absent from those of the 

 Gymnosperms. "In order to compensate, therefore, for the lack of an 

 efficient conducting system in the leaf, recourse has been had to the de- 

 velopment of these peculiar tracheides (often accompanied by bast-cells of 

 similar shape), now known as ' transfusion tissue.' In Cycas and many 

 species of Podocarpue, in which the broad pinnae or leaves are traversed 

 by a single bundle, in addition to the normal transfusion tissue, a new 

 and accessory system has been developed, running from the bundle to 

 the margin of the leaf. This, however, ... is a purely secondary 

 modification of the mesophyll-cells, and bears only a functional relation 

 to "the normal transfusion tissue, having therewith no homology whatever. 

 In the pinna of Stangeria a dichotomizing system of closely placed veins 

 springs from the large central midrib. In the pinnae of all other Cycads r 

 and in Podocarpus nageia, Dammara, and Ara/ucaria, among Conifers, a 

 system of parallel venation prevails, and here transfusion tissue is markedly 

 developed. The leaves of most Conifers are very narrow, and are traversed 

 by a single bundle, which, in all cases, is provided with well-developed 

 transfusion tissue. Ginkgo differs widely from all other Conifers in having 

 a dichotomizing system of ' bundles traversing its large, fan-shaped leaf, 



