MECHANISMS FOR CONVEYANCE TO AND FRO. 



471 



matous cells of the vascular bundle sheaths which, in summer, had been used for 

 conductino- away materials, are then crowded with starch-granules, the pores of the 

 sieve-plates are closed up during the winter; the sieve-tubes, laticiferous tubes, 

 bundle sheaths, and medullary rays do not again commence their activity until the 

 next period of vegetation, when everything becomes liquefied, and the green cells 

 ao-ain form fresh carbohydrates. These structures then serve again, of course, 

 chiefly as conducting organs. 



With regard to the junction of the conducting organs with the green cells, we 

 have a very great variety, but the many different contrivances may be gi'ouped into 



Organs for Removal of Substances. 



Laticiferous tubes from the leaf of Lactuca virosa ; x 250. 2 Vessels with spirally thickened walls, surrounded by the 

 bundle sheath, from a leaf of Ricinus communis ; x 210. 



two chief forms, viz. where the junction is direct, and where it is effected by means 

 of specially interpolated cells. 



In the first group, the switch shrubs are first to be noted, in which the foliage is 

 entirely or almost entirely absent, and where the main portion of the green tissue 

 is developed in the cortex of the rod- shaped branches, as, for example, in Cytisus 

 radiatus and in the Broom (see figs. 69 ^ 69 ^ 81 \ and 81 2). Here the ring of 

 vascular bundles forming the framework of the whole branch is surrounded by a 

 common bundle sheath, and the cells of the green tissue in the cortex are connected 

 on one side with the epidermis, and on the other with this bundle sheath, to which 

 the organic materials produced are given up directly. In the foliage-leaves of 

 many liliaceous plants, especially in the equitant leaves of irises, the green cells 

 are elongated transversely, forming a kind of bridge stretched between the vascular 



