ANGIOSPERMJE 



377 



Venation. Besides the simple parallel venation usually found, 

 there is sometimes a true reticulate venation, much like that in the 

 Dicotyledons. Such reticulate venation is found in many Araceae 

 (e.g. Anthurium, Symplocarpus), in Smilax, Dioscorea, Lilium cordi- 

 foliam, and others. In the Scitamineee (e.g. Canna, Maranta, Musa, 

 etc.) the very large leaves have a strong central midrib, with lateral 

 parallel veins running to the margin. Many Helobiege (e.g. Sagit- 

 taria) are somewhat intermediate in character, the radiating parallel 

 veins being connected by lateral ones. The leaves are usually 

 smooth, with a shining surface, or covered with a waxy bloom (e.g. 

 Agave). Hairs sometimes occur (Cypripedium spectabile), and in the 

 epiphytic Bromeliacese there are formed peculiar epidermal scales, 

 which collect moisture as it falls upon the leaves. 



Histology of the Leaf 



In upright linear leaves, such as those of many Liliaceae, the dorsiventral 

 character of the leaf is not clearly indicated by the tissues. The epidermis is 

 alike upon both sides, and stomata are equally developed. No palisade-paren- 

 . chyma is present, and the mesophyll is uniform throughout. Where the leaves 

 are broad and placed horizontally the tissues are arranged as in the leaves 

 of Dicotyledons, and the stomata are more abundant upon the lower side. In 

 xerophytic forms, like Yucca and Agave (Fig. 347, A), the epidermal cells have 

 thick walls, and a layer of thin-walled hypodermal cells lies between the com- 

 pact palisade-parenchyma and the epidermis. The epidermal cells (Fig. 347, B) 

 are usually elongated, sometimes with undulate walls, and in many cases acces- 

 sory cells are developed around 

 the stomata. These are very 

 marked in the Grasses, and in 

 other forms like Canna and 

 Tradescantia. 



Scale-leaves. Scale- 

 leaves are developed in 

 many bulbs, and upon the 

 stems of such saprophytes 

 as Corallorhiza or Cepha- 

 lanthera, and in Aspara- 

 gus and similar forms 

 where the foliage leaves 

 are replaced by phyllo- 

 clades or green branches. 

 Bracts occur in connection 

 with the inflorescence, 

 and may be very conspic- 

 uous. Such showy bracts are the spathes of many Araceae and the 

 brilliantly colored bracts of some Bromeliaceae (Tillandsia, Bilber- 

 gia) and Scitamineae (Heliconia, Zingiber). 



FIG. 348. Zannichellia palvstris. Longitudinal 

 section of root-apex (X200) ; pi, plerome; pb, 

 periblem; d, dermatogen; cal, calyptrogen. 



