MUTUAL ACCOMMODATIONS BETWEEN PLANT ORGANS. 157 



and tendril-like. In other methods of climbing, stems are more 

 analogous to foliar climbers. 



While green foliar organs are normally concerned with 

 assimilation, any green stem can assimilate as well, and in 

 some cases all foliar appendages are completely arrested, the 

 stem entirely replacing them, as in cacti, euphorbias, and 

 stapelias. These three groups, moreover, strongly mimic one 

 another in their forms, since by growing under similar arid 

 conditions they have acquired a similar outward physiognomy. 



Lastly, just as roots can become parasitic, so can stems, as in 

 Cuscuta or dodder, w T hich fixes itself to branches of clover, 

 furze, &c, and then lives upon the juices of the host-plant. 



Stipules. — These, as is shown by the origin of their fibro- 

 vascular cords, are really parts of the leaf to which they belong. 

 They assume a variety of forms and uses. Thus, they are 

 foliaceous in the pea and Galium, compensating for the loss of the 

 leaflets in the former plant ; these latter organs assuming the 

 structure and use of tendrils. They may be spinescent (acacias), 

 or like scarious bracts, protecting the buds from excessive heat in 

 some desert plants (Polycarpon), or they may constitute bud- 

 scales (lime, elm, oak, &c). 



Leaves. — The normal function of the blades of these im- 

 portant organs is, of course, assimilation, but it can be assumed 

 by the petiole (acacias) and by any of the other foliar appendages, 

 provided they be green — e.g., stipules (pea), bracts (hellebore), 

 calyx and pistil of most flow r ers. 



Since protoplasm pervades every part of a plant, we can 

 understand the possibility, though without explaining the 

 immediate causes, that leaves can give rise to buds and roots 

 and so propagate the plant. This is normally done in many 

 cases, as in ferns, Bryophyllum, Cardamine, &c; while the 

 gardener makes great use of this property in gloxinias, begonias, 

 &c. 



As stems can climb, so do leaves, either by aid of the sensi- 

 tiveness in petioles {Clematis), by the apex of the blade (Gloriosa), 

 or by the conversion of leaflets (partly) into tendrils (pea), and in 

 Lathyrus Aphaca (entirely). 



The remarkable feature of the leaves of certain plants in 



