366 BowMAN: MECHANICAL TISSUE IN CERTAIN VINES 
adaptive line of reasoning since the vine habit is most prevalent 
in habitats of diffuse light and plenty of water. As Schimper 
(7, p- 309) says, the origin of both lianas and epiphytes is to be 
traced to a low intensity of light and an abundance of moisture. 
But the lack of adequate mechanical tissue is not well accounted 
for. To attempt to explain this it seems to be necessary to look 
at the plant characters from a genetic point of view. Clearly the 
amount of mechanical tissue is a unit character in a plant and any 
variation in this direction affects this character. In the course 
of the formation of the ancestral germs of plants showing this habit 
there must have occurred a dissociation of characters and this 
segregation of characters associated together in the normal an- 
cestor gave rise to their present form; since Bateson (2) says 
segregation thus defines the units concerned in the constitutions of 
organisms and provides the clue by which an analysis of the complex 
heterogeneity of living forms may be begun. Right in the line 
of this investigation is the peculiar phenomenon observed by 
de Vries in his Oenothera cultures in the specific example of 
Oenothera rubrinervis, which has among other characters bast 
fibers with thin walls. According to de Vries’s observations 
O. rubrinervis arose once in every thousand seedlings and twelve 
times in cultures not in the direct line of descent, i. e., from the 
pure O. Lamarckiana family. In this instance de Vries thinks that 
if the group of rubrinervis characters could be dissociated, then its 
compound nature would be disclosed. How the unit can cause 
the bast walls to become thin cannot be explained, but he insists 
that the habit of a species can be so much altered by mutation 
that during its whole life and in every organ it differs from its 
parent species (8). Now, just as this could happen in Oenothera 
rubrinervis, could there not be a similar segregation and recom- 
bination of different characters so as to cause a decrease in the 
mechanical tissue of any plant in a period of mutation? 
With this question in mind an examination of the stems of 
various plants in diverse families found in the temperate zone was 
made to see if there was a warrantable deficiency in mechanical 
tissue development to account for the inability of these plants to 
stand erect. This group of plants was composed of the following: 
Rhus Toxicodendron, var. radicans (L.) Torr., Psedera quinque- 
