700 LIFE : OUTLINES OF GENERAL BIOLOGY 



Very important are the green "guard-cells" which occur in pairs 

 with a minute opening or "stoma" leading into the internal cavities 

 of the green leaf or green stem. They regulate the diffusion of gases 

 and of water- vapour. 



(2) Fibro-vascular Bundles. — When we break across the leaf-stalk 

 of the Broad-leaved Plantago, so common by the wayside, and pull 

 the two parts asunder, we see substantial longitudinal strands which 

 are continuous with the midrib of the leaf. These are the fibro- 

 vascular bundles which run through the body of all higher plants. 

 While indicated at lower and older levels, they first come to their 

 own in the Vascular Crj'ptogams, such as ferns. In these, as in 

 Flowering Plants, they consist of two portions, the wood or xylem 

 and the bast or phloem. The wood consists of elongated cells or 

 tracheids, of vessels or tracheae, and of wood-parenchyma; it has 

 chiefly to do with the transport of soil-water from the roots to the 

 leaves, and also with support. The phloem consists of vessels called 

 sieve-tubes, with perforated transverse partitions (sieve-plates) at 

 frequent intervals, and of cells more or less parenchymatous. There 

 are, of course, great differences in detail in different kinds of bundles, 

 and in their relations to one another in the stem and in other parts 

 of the plant. They end in the fine strands or veins of the leaf. The 

 bundles are developed from strands of persistent embryonic cells 

 (meristem), and some of this ever-youthful tissue may persist in 

 the bundle, continuing to add to the xylem and to the phloem. 

 This is seen in the "open" bundles of Gymnosperms and Dicotyle- 

 dons, whereas in Monocotyledons and most Pteridophytes the 

 meristem is early exhausted, and the finished bundle is said to be 

 "closed". The phloem has to do with the transport of part of the 

 proteins, but some of these seem to move in the young wood. It is 

 believed by some botanists that the phloem elements also serve to 

 distribute hormones. 



(3) The "fundamental tissue" includes all that is enclosed by the 

 tegumentary layer and traversed by the bundles. It is a collective 

 term for such tissues as cortex and pith, mainly parench3anatous. 

 It helps in support, in conducting materials, in storing reserves and 

 waste products, but in the interior of the leaf it reaches its acme of 

 function in being the photosynthetic tissue. Underneath the upper 

 epidermis this "mesophyll" consists of closely packed cells (the 

 palisade-parenchyma), elongated at right angles to the surface and 

 very richly provided with chlorophyll. It is the chief seat of photo- 

 synthesis. Below the palisade cells, and extending to the epidermis 

 of the under surface, where most of the stomata usually lie, there 

 is looser tissue (the spongy parenchyma), composed of more irre- 

 gularly shaped cells, with less chlorophyll, and with numerous inter- 

 cellular spaces. It has largely to do with facilitating the diffusion of 

 gases and the outgo or transpiration of water- vapour. 



