72 TEXTBOOK OF BOTANY 



vertical cells, the palisade layers; and the lower have irregular cells 

 more or less parallel to the leaf surface forming the spongy layers. Among 

 these cells the vascular bundles — the veins — form a network extending 

 to every part of the leaf. The midrib and larger veins increase in thick- 

 ness and may become several times as thick as the blade. Each vascular 

 bundle consists of xylem and phloem tissues surrounded by one or more 

 cell layers of bundle sheath. In the larger veins and petioles of leaves 

 the xylem and phloem may be surrounded by heavy-walled fibrous 

 sclerenchyma,^ or the sclerenchyma may be massed above and below the 

 vascular tissues, as in many grasses. In the smaller veins the bundle 

 sheath consists of thin-walled cells. So completely do the veins penetrate 

 all parts of the mesophyll that any one mesophyll cell is rarely more than 

 a few cells away from a vein. 



The xylem consists of elongated cells, and of tube-like vessels that 

 originate by a disintegration of the cross walls in strands of several or 

 many successive cells. The xylem of a single vein usually contains sev- 

 eral of these tube-like vessels — hence the name vascular bundle. The 

 movement of water to the mesophvll occurs mostly in these vessels. 

 The spiral thickening of vessels in the leaves of many plants, such as 

 agapanthus or geranium, is readily seen when segments of the blade 

 are pulled apart. Petioles of nelumbo are also excellent material for this 

 demonstration. 



The phloem tissue is composed of elongated small cells which differ 

 from those of the xylem in that they retain their protoplasmic content at 

 maturity. Foods move from the mesophyll cells through the phloem 

 tissue to other parts of the plant. 



The petiole. The petiole connecting the blade of the leaf with the 

 stem consists essentially of one or more vascular bundles and associated 

 libers embedded in parenchyma tissues. The bundles are continuations 

 of the vein system of the blade, and at the basal end connect with the 



^ The names of several common plant tissues are formed from the Greek word enchyma 

 preceded by a descripti\'e prefix. Early plant anatomists held that all tissues are deri\'ed 

 from a "fundamental" soft tissue, or parenchyma, a general term still used for thin-walled 

 tissues such as occur in many edible fruits, pith, and the soft parts of lea\'es, stems, and 

 roots. The term sclerenchyma is applied to hard tissues, such as the thick-walled fibers 

 of wood, flax, and hemp; and to the rounded grit cells such as occur in pear fruits and in 

 the bark of many trees. When mature, sclerenchyma cells die, and only the heavy walls 

 persist. Chlorenchyma is any tissue containing chlorophyll, whether in leaf, stem, root, or 

 other part of the plant. The term coUenchyma is applied to soft tissue in which the cell 

 walls are thickened irregularly, especially at the angles. These cells retain their protoplasm 

 and may be rounded or elongated. They occur in thick \'eins of leaves, in petioles, and in 

 herbaceous stems. Mesophyll (middle leaf) consists of parenchyma; it may be properly 

 called chlorenchyma only when it is green. 



