Reference for Eruvin 68:8
ההוא פולמוסא דאתא לנהרדעא אמר להו רב נחמן פוקו עבידו כבושי כבשי באגמא ולמחר ניזיל וניתיב עלויהו
if IT HAD BEEN UPROOTED AND THEN INSERTED [IN THE GROUND is the 'erub effective, [from which it follows, if it was] not first uprooted and then inserted [in the ground the 'erub is] not [effective]?<span class="x" onmousemove="('comment',' Obviously because it is forbidden to use a growing reed. How then could R. Nahman permit the use of an embankment made of growing reeds?');"><sup>23</sup></span> - The other replied: There<span class="x" onmousemove="('comment',' The ruling in our MISHNAH:');"><sup>24</sup></span> [it is a case] of hardened reeds.<span class="x" onmousemove="('comment',' Which are regarded as trees the use of which on the Sabbath is forbidden. Soft reeds, however, which come under the category of herb, may, therefore, be used.');"><sup>25</sup></span> And whence is it derived that we draw a distinction between hardened, and unhardened reeds? - From what was taught: Reeds, thorns and thistles belong to the species of trees and are not subject to the prohibition of kil'ayim<span class="x" onmousemove="('comment',' V. Glos.');"><sup>26</sup></span>