CHAP. II. 



EXOGENOUS STEMS. 



79 



In Stauntonia latifolia 

 {Jig. 37.), which has a 

 twining stem, there are 

 no concentric circles, and 

 the meduUaiy rays ai-e 

 curved, part from right 

 to left, and part from left 

 to right, diverging at one 

 point and converging at 

 another ; the bark is pier- 

 ced with extensive lonjri- 

 tudinal perforations. 



In Euonymus tingens {Jig. 

 38.) the vessels near the centre 

 of the stem are arranged in con- 

 centric interrupted circles, but 

 towards the bark there is no 

 trace of such circles ; the sm-- 

 face of the stem is deeply cut 

 into lobes paiallel with the 

 stem, and the vessels are all 

 confounded in an uniform mass. 

 Gaudichaud represents the stem of some Malpighiaceous 

 plants to be in like manner divided into a number of regular 

 lobes, which, however, actually reach the axis ; and, in con- 

 sequence of the twining habit of the stem, are twisted into the 

 appearance of a cable externally. 



In Menispermum laurifolium {Jig. 

 39.) the concentric lines evidently 

 belong to the medullary system ; they 

 are extremely interrupted and un- 

 equal, often only half encircling the 

 stem, or even less, and they anasto- 

 mose in various ways ; the medullary 

 rays are unusually large, and lie across 

 the wood hke parallel bars; and, 

 finally, the plates of which the wood 

 consists each contain but one vessel, 

 which is situated at the external 

 edge of the plate. 



