PLATE XXI.— CROSS SECTIONS. 



A, PiNUS T/EDA, V"- '■• fl-t resin Uuct ; «. c, secretiug cells; wi. r., medullary rays; a, section of transverse tracheid of 

 ray; h, the ray leaves tlie plain of tLe section at this point, small parts of it reappearinj; further on; c, simple 

 pita connecting parenchyma cells of the ray; d-e, part of a row of tracheids formed during one season; 

 /, flattened terminal part of a tracheid. 



Ji, PiNUS HETEUOPIIYLLA, ■'^'i'". ap. )('., springwooil ; mi. »'., summerwood ; a-}i, part of a niw of traclirids fcirmcd 

 during one season; r, tcnninal i)arts of tracheids; d, bordered pit in spriugwood; f, same in siininiiTuood; 

 other letters as in A. 



C, PiNUS GLABRA, '^". c, TOW oC tiaclieids doubled; other letters as in B. 

 Originals, all ^V^ 

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