THE STEjSI 



97 



v::i 



forniing the transverse walls of two superposed sieve-tubes. 

 In Fig. 1 1 is shown a longitudinal section through such a sieve- 

 tube and its sieve-plate. The pores on the left-hand side are 

 still filled with the protoplasmic contents, while on the right 

 hand the action of the alcohol has caused them to shrink and be 

 withdrawn from the pores. They have, however, been hardened 

 by the alcohol in the form of 

 finger-like processes, which at one 

 time penetrated the plate. 



It is not without ground that 

 we have emphasised the fact that 

 these groups of soft bast run on 

 the inside of the cortex parallel to 

 the strands of woody tissue. They 

 are, in fact, a part of these fibro- 

 vascular bundles which traverse 

 the stem. 



With this knowledge we are 

 now able to picture to ourselves 

 the structure of a stem or branch 

 about one year old. If we con- 

 sider such a shoot to be placed 

 vertically upright before us, it will 

 be found to be an elongate struc- 

 ture consisting of many stories. 

 Every story is marked by the in- 

 sertion of a leaf. The chief form 

 of brick in this tower-like struc- 

 ture is the parenchymatous cell, 

 which can best be compared with 

 the cell of a honeycomb. Of such cells the leaves and the 

 shoot are mainly built up. Every parenchymatous cell is a 

 small chamber, which shows at a certain time of its life a great 

 activity in forming new organised material. Such activity 

 can only take place to any extent if new raw material is 

 constantly brought to it and the assimilated substance is con- 

 stantly removed from these manufacturing cells. For this pur- 

 pose special passages are formed, the fibro-vascular bundles, 

 which consist of two chief portions. One part consists of 



(f 



I'iG. II.— Longitudinal Section 



THROUGH A LARGE SlEVE-TUBE OF 



Lagenaria vulgaris. 



Its protoplasmic contents have been 

 contracted by alcohol into the strand c. 

 The protoplasmic processes which passed 

 through the pores g have been hardened, 

 and have shrunk away from the sieve- 

 plate {after De Bary). 



