248 Universliji of Calif ornm I'Khlicatiotis in Bota)nf \\'o\..7 



is need of fiii'llu'i- oxpoi'iiiiciits mikI ;iiialyses to continn tlic I'csiilis of 

 tln' iiioi-c ur less prcliniiiiary experiments tabulated above. Iiub'cd a 

 few ex('e|)tioiis to our f^eiiora] conelusion appear in labU" 9 and while 

 not mmii'rous eiiou<^li to att'eet it materially eertaiidy cast some doubt. 

 n|>oii it. It is entir<'ly possiltle that in selectiufr tlie two dates for 

 i-olleetion of sam|)l('s the critical jx-riod may liave been overlooked. 

 Thus, if a sample had been taken in August or earliei" as well as in 

 Septemlx'i' a decided iiicrease in ?-nbber content mij»ht have been 

 noted for the lattci" date. This would have meant that tlie flowering 

 period in Chnjsothamnvs represents, in terms of seasonal variation 

 in rubber content as observed for Guayule. not tlie eiul of the grov^'ing 

 ])eriod but the beginning' of the resting period tlui'ing the start of 

 which maximum deposition of rubber takes place very rapidly.-'' Had 

 such collections been inade and such results obtained no light wo\d(l. 

 however, have been thrown upon the seeming disappearance durin>i- 

 the i-esting period of a certain proportion of the nd)ber present within 

 a month of its start. 



XI!. METHODS OF HARVESTING: SEASON. AGE, DEPTH 



OF CUTTING. ETC. 



The methods of harvesting have not been woiked out in detail since 

 it has not been demonstrated that Chrysothannnis can be utilized on a 

 commercial scale. However, our preliminary studies have indicated 

 certain general principles to be observed in case rubber is ever prepared 

 from the plants and they are here set forth as follows. 



The question of the distribution of rubl)er in the various plant or- 

 gans and the extent of its occurrence in stems of various ages is dis- 

 cussed elsewhere (cf. p. 234). It may here be said that the amount of 

 rubber in i)arts less tluui three years old is relatively small. From the 

 thii'd year onward for one oi- two years the amount increases rather 

 I'apidly. After the fifth or sixth year the (piantity of i'id)ber ])i'esent 

 remains more or less constant. The cause of this constancy in amount 

 depends upon certain facts taken up in detail elsewhei-e. In the first 

 place, the extracambial tissues ai'c almost the exclusive i-egions of lub- 

 ber deposition. Secondly, when cork fornuition takes place there is. 

 each yeai". an amount of nd)ber containing cortical tissue cut off whieli 

 roughly appj'oxinuites the amount of new cortieaj tissue aniuudly 



-" The microscopical evidence (cf. j). 240) appears to give a partial confirinatioii 

 of such a supposition. 



