TISSUES AND SIMPLE ORGANS. 



75 



represents their appearance during the summer. A noteworthy 

 change takes place in these sieve-plates during the winter: the 

 entire sieve-plate septum becomes swollen 

 and softened on the two surfaces, similar 

 to a gelatinous cell-membrane (" callus "). 

 This callus evidently serves to close the 

 openings of the sieve-plate in the winter. 1 

 The following table gives the earlier and 



fou 



ter names <> f "ell-forms which were in- 

 eluded in the old expression " vascular 

 bundle." It forms a fitting conclusion to 

 the discussion of the cell-forms enumerated in this chapter. 



sieve-plates. 



HABEKLANDT'S Synopsis of Cell-forms, Slightly Modified by the 



Author. 







Bast and libriform really do not belong to the 

 they form (inclusive of collenchyma) an independent tissue- system, 

 the stereome (mechanical system). 



A small conducting system which is not widely distributed, but 

 is limited to certain plant-families, has not yet been touched upon, 

 but will now be briefly discussed. This is the laticiferous tissue 

 (" milk-tubes").' 



1 DE BARY, Comparative Anatomy. 



