53° 



ECOLOGY 



and it may be that the red pigments are merely the indices of certain chemical 

 activities that are quite without functional significance. Various annuals {e.g. 

 Chenopodiuni) assume vivid colors just as they are dying, and any advantage in 

 such coloration is most improbable. It is not unreasonable to suppose that plants 

 may have many useless organs and characters, which are mere by-products of their 

 fundamental activities. 



2. THE STRUCTURE AND ARRANGEMENT OF CHLORENCHYMA 



Structural variations in chlorophyll-bearing tissues. — Mesophytic 

 dicotyls. — The tissues that contain chlorophyll, i.e. the chlorenchyma, 

 show considerable diversity, referable in large part to environment. 

 Most mesophytic leaves lack epidermal chlorophyll (except in the guard 

 cells of the stomata), but most of the internal cells except those in the 

 conductive tract contain chlorophyll, more being present in the upper 



than in the lower half. 

 The cells of the upper 

 portion, known as pali- 

 sade cells (or, simply, as 

 palisades), are elongated 



Fig. 760. — A cross section of a mesophytic leaf, 

 that of a peppergrass (Lepidium), showing the upper 

 epidermis (e), the lower epidermis (e'), stomata (5), the 

 chlorenchyma (c) consisting of closely placed palisade 

 cells (p) and more loosely placed sponge tissue (/), 

 and a vascular or conductive tract (v) with bundle 

 sheath (6), hadrome or xylem (A), and leptome or 

 phloem (/); considerably magnified. 



Fig. 761. — A cross section 

 of a mesophytic leaf, that of 

 a blue violet {Viola cucullata)^ 

 showing a single row of greatly 

 elongated palisade cells {p), 

 beneath which is a region of 

 loose spongy tissue (/) ; con- 

 siderably magnified. 



transversely to the leaf surface (especially in dicotyls) , and are arranged 

 in one to three compact layers (figs. 760, 761). The cells of the under 

 portion, the spongy parenchyma (or, simply, the sponge), are arranged 

 loosely and irregularly and have prominent intervening air spaces, 

 the lacunae. Both the palisade cells and the sponge cells have thin, 

 permeable, cellulose walls. 

 Mesophytic monocotyls ; hydrophytes and shade plants. — The leaves 



