650 THE POPULAR SCIENCE MONTHLY. 



probability is, that the plant or animal will continue to develop in that 

 direction till it diverges widely from the original form. The struggle 

 for existence will cause all the imperfect forms to be killed off, and 

 only those will survive which are best suited to the altered conditions 

 of life. Once let an organism begin to vary in any one direction, 

 and there is no telling Avhere or when it will stop. This much ia 

 certain, that it never ceases until the best results possible have been 

 attained. 



The chief characteristic, then, of the convolvulus family is the 

 climbing habit. The origin of this habit is found in the fact that 

 sunlight and air are two things needful for a plant's proper growth 

 and development. In situations where these two things are found in 

 limited quantities, plants with climbing habits and animals with arbo- 

 real instincts will abound. In Brazil, for instance, where immense 

 tracts are covered with a dense forest-growth, it is noticed that all 

 forms of animal life have become adapted to residence in trees. Many 

 of them live there entirely. Monkeys seldom leave the tree-toj^s. Liz- 

 ards and snakes and insects are there, and even man himself is often 

 found living among the branches. So, too, plants form immensely 

 long stems, reaching in many cases to the tops of trees a hundred feet 

 high. The extraordinary development of climbing powers has been 

 gradually acquired in the course of ages. In times and places where 

 vegetation v/as not dense, and where the struggle for light was not 

 great, plants of erect habit succeeded well. Then it was a conflict to 

 see which could grow tallest. But when a weak plant found that, by 

 taking hold of its tall and erect neighbor and by clinging to it, it could 

 reach the sunlight much easier and by an expenditure of much less 

 material than by growing erect itself, it was a great step on the road. 

 This habit, being transmitted from one generation to the next, kept on 

 improving. Less and less rigid, more and more flexuous stems ensued, 

 and the delicate climbing vines of modern times are the results of this 

 necessity of reaching sunlight with as little waste of material as pos- 

 sible. 



There are many methods adopted by plants to climb. "While some 

 of them reach upward by means of tendi'ils developed at the ends of 

 stems or leaves, others twist their petioles round the sujDport, and still 

 others twine their stems round other stems that may come in their 

 way. This last is the method adopted by those of the Convolviilacece 

 which climb at all. For even in this family there are some sjDecies 

 which are erect in growth. The Cahjstegia spithamcea is one of 

 them. Others do not grow up into the air, but trail along the ground 

 or over low plants, and thus secure their due share of sunlight. Oth- 

 ers, again, climb freely, and this is the case with the dodder. 



The climbing bitter-sweet is said to sometimes strangle the trees 

 upon which it grows. The constriction caused by its growing stem is 

 so great as to cut off the supply of sap from the roots, and cause the 



