AN OUTLINE OF PHYTOBIOLOGY. 17 



3. By extreme minuteness a relatively large surface propor- 

 tional to bulk can be f^ttained without special appendages, for 

 as a sphere diminishes in size its bulk diminishes relatively more 

 rapidly than its surface. Thus very minute spores can swim in 

 the air like dust, as do these of moulds, etc., and those of mosses, 

 ferns, etc., can easily be blown to great distances. Amongst 

 Phanerogams the presence of a several-celled embryo makes it 

 difficult to reduce them to so small a size, but in certain orchids 

 they are very minute. Plenty of plants bear tiny seeds, which, 

 thrown from their capsules by strong wind-gusts, are by them 

 carried considerable distances, as will be mentioned below. 

 Sometimes as in orchids, the seeds are surrounded by a sort of 

 loose sac containing air which makes them much lighter. 



4. Seeds or fruits of trees develop wings which either act 

 as sails or else as parachutes, causing the slow fall and hence 

 wider lateral carrying by the wind. In all cases the centre of 

 gravity of the seed is so adjusted that the fall is as slow as 

 possible. In the simplest cases, the seed itself becomes very 

 flat and thin, as in some species of Iris ; in addition there may be 

 the development of a wing around the edge. The wings may 

 be formed from a bract as in linden, grasses, Carpinus ; from the 

 ovary as in elm, maple, ash ; from the corolla in rare cases ; 

 from the calyx as in scabiosa ; from the seed-coat as in catalpa, 

 pines and spruces. 



As noted, wings are almost confined to tree seeds, since the 

 height of the tree gives them a good start with the winds, which 

 would not be true in herbs or low plants. 



5. Upon herbs and low growing woody plants, where the 

 seeds need not only to be carried laterally but also raised in the 

 air, plumes or tufts of hairs are developed. Frequently these 

 are such that the entire mass forms a light ball, but more often 

 the plumes are at one end, sometimes connected with the seed 

 by a stalk, as in the dandelion, in which case the whole structure 

 keeps an ui^right position and the wind may carry it for immense 

 distances. Indeed where the plume spreads out horizontally 

 and the seed hangs on a stalk beneath to keep the whole 



