HISTOLOGY OF ANNUAL DICOTYL STEM 



23 



Scaly bulbs have narrow imbricated scales, the outer ones not en- 

 closing the inner. Ex. : Lily. 



Tltbers and corms are annual. Bulbs and Rhizomes are perennial. 



Exogenous and Endogenous Stems. — Exogenous stems are typical 

 of Gymnosperms and Dicotyledons and can increase materially in thick- 

 ness due to presence of a cambium. Such stems show differentiation 

 into an outer or cortical region and an inner or central cylinder region. 



Endogenous stems are _ 



typical of Monocotyledons .««(^^^^^^^5llBfc^ ^ 



and cannot increase mate- ^4lr7^||^^SjH^|^|£^^ ]-) 



rially in thickness due to ab- 

 sence of cambium. Such 

 stems show no' differentia- 

 tion into cortical and central 

 regions. 



Histology of Annual 

 Dicotyl Stem. — (In both an- 

 nual and perennial dicotyl- 

 edonous stems endodermis 

 and pericambium are rarely 

 seen since each has become 

 so similar to cortex through 

 passage of food, etc.) 



1. Epidermis, cutinized, 

 with hairs. 



2. Cortex composed of 

 three zones : an outer or exo- 

 cortex, whose cells are thin 

 walled and contain chloro- 

 plasts; a middle or medio- 

 cortex, consisting of cells of 



indurated walls giving extreme pliability and strength, an inner or endo- 

 cortex, a very broad zone of thin and thicked-walled parenchyme cells. 



3. The innermost layer of cells of the cortex called the endodermis. 

 (Not generally distinguishable.) 



4. Pericambium. (Not generally distinguishable.) 



5. Fibrovascular bundles of open collateral type arranged in a 

 circle with primary medullary rays between the bundles. 



6. Pith. 



Fig. 17. — Photomicrograph of cross-section 

 of stein of Aristolochia sipho, where cambial 

 activity is just beginning, a, Epidermis; b, 

 collenchyma; c, thin-walled parenchyma of the 

 cortex, the innermost cell layer of which is the 

 starch sheath or endodermis; d, sclerenchyma 

 ring of the pericycle; e, thin-walled parenchyma 

 of the pericycle; /, primary medullary ray; g, 

 phloem; h, xylem; i, interfascicular cambium; j, 

 medulla or pith. X20. {From Stevens.) 



