66 ECESIC CAUSES. 



migration of free parts, such as spores, seeds, and fruits, especially in large 

 areas. Naturally, species which are readily carried by seeds and fruits, and 

 move also by offshoots, form excellent pioneers. The influence of size of organ 

 is indicated by the relative mobility of spores, seeds, and fruits. In spite 

 of many exceptions, spores are more readily and widely distributed than seeds, 

 and seeds than fruits. This is shown in some measure also by the success in 

 migration of plants in which the fruit simulates a seed almost perfectly, as in the 

 grain, achene, etc. The handicap of the fruit in regard to size is often counter- 

 balanced by the perfection of the contrivance for dissemination. In the case of 

 tumbleweeds and tumbling grasses, the whole plant or the major portion of it 

 has assumed a form which amounts practically to a nearly perfect contrivance 

 for effective migration (plates 14c, 15a). 



Influence of the migration contrivance. — The effect of the modification for 

 carriage is intimately blended with that of the agent, as would be expected. 

 The perfection of the device determines the success of the agent, as is well 

 seen in those modifications which increase the surface for wind carriage. 

 Sack and bladder fruits, as in Physalis, are relatively ineffective, and are 

 often associated with other devices. Wings give greater buoyancy, but are 

 only moderately efficient, except when the seed or fruit is small and light. The 

 vast majority of samaras of the elm, maple, ash, etc., fall near the parent tree. 

 This is strikingly true of the seeds of conifers. A careful transect study of the 

 flight of seeds of the spruce and the fir showed that practically all of them 

 landed within a distance equal to the height of the tree. Comate and pappose 

 seeds and fruits are by far the most efficient of wind-borne disseminules, and 

 probably of all kinds as well. Here again success is determined largely by 

 smallness of size, but apart from this the perfection of the device as to the 

 number, length, and position of scales or hairs is decisive. Scales are less 

 efficient than bristles or hairs, and the latter are successful in proportion to 

 length and number. Disseminules tufted at one end are carried more readily 

 than those covered with hairs, and a pappus which spreads widely or is plumy 

 is the most effective of all. The relative efficiency of devices for carriage by 

 animals is less evident, but the number of pioneers which possess fruits with 

 spines or hooks is significant. 



Many fruits migrate readily, even when the migration device is not greatly 

 perfected. This is due to the fact that they avail themselves of two or more 

 agents, either by means of two distinct devices or because of their behavior on 

 drying. In Physalis the bladdery fruit is rolled over the ground by the wind, 

 and then the seeds scattered by birds and rodents. Stipa, Erodium, and other 

 plants with sharp-pointed twisting fruits, are carried by attachment and 

 blown by the wind in tangled clusters, the two agents often alternating many 

 times. A striking case of this sort is afforded by Micrampelis, which is a 

 frequent pioneer in denuded areas along streams. The fruits are blown by the 

 wind, floated by streams, and even carried by attachment, while the seeds, 

 in addition to being forcibly expelled, are readily carried by water. 



The distance of migration is a direct consequence of the perfection of the 

 device. Hence the latter is of the first importance in selecting the migrants 

 which are moving toward a new area. It thus plays a large part in determining 

 what species will enter it as pioneers, as well as the stages in which others will 

 appear. The comate seeds of fireweed, aspen, and willow may be carried for 



