SUNFLOWER STEM 61 



^^ : 



bundle (more or less distinctly double), and four lateral bundlesj 

 two on either side. 



2. The lateral pairs coalesce more or less completely, so 

 that on entering the stem the leaf-trace consists of three 

 bundles. 



3. The lateral bundles of the two opposite cotyledons coalesce 

 after their entry into the stem, so that the whole trace of the two 

 cotyledons together appears as four bundles in the lower sections 

 of the series. 



4. The vascular system from the plumule appears as a ring of 

 bundles of indefinite number. 



5. The four bundles of the leaf-trace from the cotyledons 

 insert themselves between the bundles of the plumule, and thus 

 enter the ring. 



The sections may be made with a microtome, but this is not at 

 all necessary in such a case as this. 



The investigation of a bundle-system as above described is a 

 most useful exercise : by thus comparing a number of sections, 

 putting the results together, and so reconstructing in the mind 

 the whole shoot from which the sections were cut, the student 

 will acquire the habit of regarding a section not as a mere net- 

 work of cell-walls, but as a slice of tissue which had a certain 

 definite position in the plant from which it was cut. This *habit 

 of constantly referring the section to its place in the plant, at 

 the same time as its details are being examined, cannot be acquired 

 too early. 



Microscopic Observations. 



The material should be kept in spirit for some time 

 to remove resin, and air, and to harden the tissues; 

 but this is not indispensable, and fresh material may be 

 used, though it is not so satisfactory. 



I. Cut thin transverse sections of a stem of a well- 

 grown plant of Helianthus, i.e. of a stem more than 

 half an inch at least in diameter. 



