PROTECTIVE STIPULES. 107 



ordinary bursting of this membranous stipule-sac is caused by the 

 starting growth of the parts of the plant which lie tightly folded 

 within it, but it is also liable to become broken up by the destructive 

 action of spring frosts. 



It is the habit of all the members of the family to which Rheum be- 

 longs to provide their leaf stalks with ample membranous stipules, 

 but in most cases at least they are of little or no apparent use to the 

 plant. An evident use of those organs to Rheum, however, is protection 

 against spring frosts, and although they are themselves thus destroyed, 

 the parts which they cover are fully saved by their intervention if the 

 frost is light. 



If Rheum should encounter a light frost just after she has raised 

 her head above ground she would not be thereby injured or disturbed 

 in any part of her season's growth, because the force of the frost 

 would be expended on the stipule-sac, which has no further use. But 

 if the frost at that, or a subsequent time should be so severe as to injure 

 or destroy the first cluster of flower buds as well as the stipule-sac 

 which covered it, another is ready to take its place as soon as the tem- 

 perature moderates. That is, the next cluster, springing from the 

 axil of the second leaf, lies unharmed beneath the injured one, 

 covered there by the stipule-sac of that leaf. Furthermore, there is such 

 a sac for every coming leaf and flower cluster. 



Thus cluster succeeds beneath cluster until the whole series of 

 leaves of the coming stem, their stipule-sacs and the axilary flower 

 clusters of each, are all numbered. Even if half of those flower clusters 

 should be successively destroyed by frost the remainder are severally 

 protected and preserved until the advent of more favorable weather. 

 These alone are amply sufficient to mature an abundant progeny for 

 the plant ; and none of the preceding disasters will have inflicted real 

 injury. 



Smilax does not need the kind of protection that Rheum requires, but 

 having a very slender stem, although a woody one, it needs protective 

 support like that required by climbing plants. The tendrils of climbing 

 annual plants are sometimes modifications of leaves or leaflets, but it 

 matters little to such plants what part is so tnodified because not only the 

 leaves but the whole plant is destroyed at the end of the season. 

 Woody plants, however, which are supported by tendrils need their 

 permanent aid ; and such tendrils must therefore not be deciduous, as 

 leaves are. 



The tendrils of woody plants need to remain on duty after the 

 leaves and fruit have fallen off, and to extend their service through 

 all the seasons of several years. This service is naturally and fully 

 accomplished in the case of those plants whose tendrils are modified 



