240 f'iiii'( rsil 11 (if (\il ifonilii I'lihllcaf ions in Ihildinj | Voi,. 7 



of {he cortex and iinii-ruiict ional plilociii iiicrv niciits. 'I'lii- t'mictioiuil 

 pliloi'iu aiH'as ai'c also nil)l)('i' l)t'ai'iiiji' altlioujili in tliis case tlic 

 protoplasmic matrices of the cells often stain in simulation of rul)ber 

 inclusions. The sieve tubes and companion cells are (piite small in 

 eross-seetional diametei- and oidy in longitudinal sections can rubber 

 lildbides be identitied within them. The ])hloein parenchyma bears 

 the lar<»'est amount of ruitber in the phloem ai'ea and even in cross- 

 sectional view thei'e is no difficulty in identifA-ing the stained inclu- 

 sions as such. 



The candiium aj^pears consistently to bear rubber inclusions in 

 stems moi'e than one year old. The rubber is in tlie form of small 

 globules which seem to increase in innnber during cambial activity. 

 As an annual cand)ium increment towai'd the j)eriphery begins to 

 become diffVi-eutiated into characteristic phloem elements these snudl 

 globules tend to agglomerate. Intracambially. however, the partially 

 differentiated xylem elements contain no rubber inclusions although 

 originating from the same nd)ber bearing cambinm which passes 

 on its rubber inclusions to the developing phloem increments. This 

 situation might furnish a starting poini for investigations on the 

 chemical or physical constitution of the j^rotoplast which is specific 

 for the differentiation of rubber inclusions or essential for the perma- 

 nence and persistence of such inclusions. In none of the mature xylem 

 elements, parenchymatous or otherwise, have rubber inclusions been 

 found. 



Under the microscope one of the most conspicuous features which 

 attracts attention when preparations such as those shown in plates 

 18 and 1!) ai-e examined is the increase in I'ubber deposition at certain 

 l^oints along the rays. Although this matter is not well brought out 

 in all the photographs it may still be seen that at points correspond- 

 ing to the spring wood of each annual xjdem increment the amount 

 of rubber is less than at those points along the rays which correspond 

 to the summei" or fall wood. This situation will be mentioned (dse- 

 where in eomiectif)!! with a discussion of seasonal vai'iation in i-ubl)er 

 content. 



Plates IS and Ifj (exclusive of figures 4 and 5) show cross sections 

 of stems of vaiious ages. The stem of the current year shown in 

 j)late 18, figuie 1, is included to call attention to the deep staining of 

 the chlorenchyma, the absence of stainable substance in cortex and 

 rays, and es])ecially the large (piantity of stained ru])bei' in the xylem 

 intrust ion of an axillary bud. The three year old stem in plate 18, 



