146 



PLANT ANATOMY. 



dary medullary rays, which occur in the 

 process of secondary growth. 



In shape tho cells of the medullary 

 rays are iso-diametric with slightly 

 tliioki'ued ri^ht-angled walls, Mhich are 

 provided with simple pores. Their func- 

 tion is to convey nourishment to differ- 

 ent parts of the plant during the grow- 

 ing' time; and during the resting period 

 of the plant they serve as storehouses of 

 reserve material, starch, etc. The num- 

 her and size of the medullary rays, the 

 sliapeand sizeof the markings of the in- 

 diviilual cells, are of diagnostic impor- 

 tau(M' for pharnmcugnostieal purposes.(a) 



Woody Parenchyma is also a product 

 of the oanihium by means of cross- 

 walled division. They are iso-diametric 

 in cross section and somewhat elongat- 

 ed. They conduct plastic materials by 

 means of their thin Avails, which are pro- 

 vided with round or oval or simple pores. 

 r>uring the resting period of the plant 

 ihese cells contain reserve materials, 

 mainly starch. 



5. SECRETORY AND EXCRE- 

 TORY SYSTEMS. In the process of 

 plant developement many compounds are 

 produced which the plant either stores 

 up for future use or gets rid of entirely. 

 .Alany of these secretions have especial 

 reservoirs adapted for their reception, 



while others use ordinary conducting 

 cells. 



A large nuniber of these secretions 

 liave no economic value in contributing 

 to the growtli of the plant, bu t may be 



rii?\v^'?M^"l^^.' Vergleiehende Anatomie 

 der ^Aichtlgste deutschen Wald nnd Onrten 

 holz arten, .Stuttgart. ISSl. 



Fig. 64.— Branched milk tubes in the cortex of 

 stem of Lactuca virosa, which contain Lactu- 

 carium. (de Bary). 



of use in attracting insects, healing 

 wounds, or in preventing the ravages of 

 aninnUs. The secretion of oil and its re- 

 lated products may have its influence 

 upon the temprrature of plants by pre- 

 serving tht'Ui from excessive heat in the 



Fig. 03. 



Cross section of resin passage in leaf of Finns; oil cells in leaf 



