its mother plant was sickly and its grandmother would have died at once. It was in recognition of this view that in the eighteenth century the botanical garden at Teneriffe was established (the so-called acclimatization garden at Durasno and the Colegan Garden at Orotava, at an altitude of 1,040 feet) in order to furnish a temporary resting place for tropical plants that they might accustom themselves to a cocler climate preparatory to their cultivation in southern Europe. According to Döllen, the same principle is applied in the acclimatization garden at Algiers to tropical African plants before their transportation into southern France.

As the guiding thought of his second memoir, Linsser now remarks that we must divide the vegetable phenomena of the world into two divisions, viz, those in which temperature controls the annually recurring cycle of phases, as is the case in the Temperate Zone, and those in which moisture controls, as in the Tropical Zone. Thus, on the grassy plains of South America, where the year is divided into a dry and a wet season, the entire course of vegetation depends upon the latter; the hottest and driest season exerts upon the vegetable life an influence like that of the northern winter, bringing, namely, rest and even death. Such a contrast is even found at Madeira, where, according to Heer, the weeds of northern Europe begin to vegetate in the fall after the dry summer months of trade winds and when the first rains fall, whereas in the hottest summer time all these weeds slumber or die, as with us in winter. In the steppes of Orenburg, Russia, when the sun melts the snow in April, it starts the first sprouts and the blossoms, and by the beginning of May the vegetation of the steppes has attained its highest brilliancy, being distinguished by the great number of many-colored tulips, as has been so often described by travelers; but this beauty passes by with remarkable rapidity, and when in June the dry, hot summer of the steppes begins, all the verdure is dry and dead, and in place of the blossoms there are seen only the dry, empty hulls; so that the whole life of the plants on the steppes is condensed into the short space of eight weeks.

We thus see that for large portions of the earth the heat as such ceases to be the principal regulator of plant life, and moisture becomes the controlling influence.

It is evident that the life of plants depends upon both temperature and moisture. In situations where there is always sufficient moisture the influence that decides whether or not a plant shall develop is the heat; but in regions where there is always sufficient heat that deciding influence is moisture. Therefore Linsser proposes in his second memoir to first state the influence of heat on vegetable phenomena more precisely than he had previously done, and then to develop the influence of moisture.

Linsser's second study is based upon a much larger mass of phenological observations than that previously used by him, and, in fact, more than has ever been used by any other investigator of this subject. The accompanying table gives for each of his stations the initial and final dates when the normal mean daily temperature is 0° C., or the date when the minimum of the year occurs if that minimum is above 0° C.; these are the limiting dates between which the summation of temperature is made according to Linsser's method. The sums total of positive temperatures for the whole year are given in the third column in centigrade degrees.

A plant has access to water by two methods-through its roots it absorbs the water in the soil, whereas its leaves come in contact with the vapor and the rain in the atmosphere; but Linsser considers that the relation of the plant to the water in the soil is the important feature that decides as to the development of the peren

nial plants of temperate regions, which are those considered in his second memoir. So he leaves the study of atmospheric vapor and plant life to the future, while confining himself at present to the relation between rainfall and the periodic phenomena of vegetation. It is not necessary to reproduce the tables of normal monthly rainfall given by Linsser for each of his stations, and generally based upon many years of observations. Of course, these numbers expressing the local rainfalls are, as is well known, less directly applicable to a neighboring locality than are the mean monthly temperatures, and they must be used with correspondingly less confidence.

The constant fractional part of the annual sum total of heat, as previously established by Linsser, afforded him a valuable suggestion or a working hypothesis as to the relation between the life of the plant and other factors, such as sunshine, rainfall, nutrition, and in fact every factor that influences the life of the plant. If, namely, a plant utilizes one-tenth of its annual cycle of heat in order to bring it to the leafing stage, why may it not also require one-tenth of its annual cycle of rain or sunshine or some similar constant fractional part? Now, in the development of a plant there is necessary, first, the material, viz, rainfall, or irrigation water with the nutrition contained therein, and on the other hand one or more forces, such as sunshine and heat, by the help of which the plant can utilize that material in its process of assimilation. The different phases of the development of the plant, such as the appearance of the blossoms and the ripening of the fruit, are work accomplished; in this work the water supplies the principal material, while the heat, says Linsser, plays the rôle of the principal force; but the work of the plant-that is to say, its progressive development-will only be in proportion to the force, so long as the latter finds a sufficient quantity of material present to insure the complete utilization of the force. Evidently a force that is competent to convert a certain quantity of material to the use of the plant will only be half utilized if only half of this quantity of material is present. In other words, the development of the plant goes on in proportion to the quantity of heat only so long as the plant has at its disposal the maximum quantity of material that can be worked over by this heat.

Therefore any further investigations as to the relation of the life of a plant to its external factors must necessarily consider the distribution of material with reference to the distribution of heat. In our present case it is the distribution of the quantity of rain with reference to the heat, and if such relative distribution is not considered then its omission is only permissible under the assumption that during the whole period of vegetation the material necessary to the growth of the plant is always present in such quantity that at any

moment the force then acting can be completely utilized. This assumption as to rainfall is actually fulfilled over by far the largest part of the European area hitherto studied by Linsser.

Of course, we can not speak of absolute quantities of heat or nourishing material. We have to do only with their relative distribution during the period of vegetation-that it to say, with the ratio of the quantity of material (f) to the quantity of heat (w). If we consider that the quantity of material that a definite quantity of heat is able to work up for the use of the plant is directly proportional to this quantity of heat, then the ratio f/w will have for each plant and phase a certain definite value that may be called the most favorable ratio and for which value the material on hand is completely used up by the heat or active force that is present. If the material that is present is not sufficient for the heat, then fe is smaller than this most favorable value, and in this case the material is completely used up; but a portion of this heat remains unused and wasted. If, on the other hand, the heat is not sufficient to use up all the material, then fr is too large and the heat is completely used, but a portion of the material is wasted.

The fractional portion of the annual sum total of heat that is needed to bring a plant up to any stage of vegetation is by Linsser called the "physiological constant" for that phase and plant, and is constant wherever the plant is acclimatized. The ratio f/w, as compiled by him month by month for each of his stations, is a local climatic constant, which is large when the climate is favorable to the growth of the plant-that is to say, when there is abundance of rain-but is small when the climate is more or less unfavorable to the plant-that is to say, when the summer rains are deficient.

The vegetation of the whole world is, according to Linsser's views, to be divided into zones (A, B, C, D, E, F), according to the annual distribution of the monthly ratios f/w. Thus in the highest latitudes (Linsser's zone A) and in the greater part of the European region covered by Linsser's researches, there is during the entire year a deficiency of heat, but a sufficiency of moisture and of material to employ all the heat force that is available. In the Steppes of Russia, however, there is a deficiency of moisture during the summer and autumn, and the fraction fr becomes quite small for the zone B. The other localities that have a wet and a dry period annually may be divided into three classes, viz, C, where the drought comes during the months of July and December; D, where the drought comes during the months of January and June, or E, where there are two annual droughts, January to March and June to August. This latter arrangement is shown in Madeira in the vegetation of certain kinds of apples. Finally, we may have in zone F a perpetual abundance

of both heat and moisture, in which case all annual periodicity disappears and the plant goes through its cycle of vegetation independent of the months of the year, as in the warm and rainy regions of Java.

As before said, the absolute value of the ratio f/w need not be considered at present, and in fact it changes with the units of time, of temperature, or rainfall, etc. Linsser divides the depth of the monthly rainfall, expressed in Paris or French lines, by the average temperatures of the respective months expressed in degrees Centigrade.

In order to ascertain which of his European stations lies in the zone A and which in the zone B it is necessary to adopt some limiting value for the ratio f/w, and to this end Linsser examines these ratios in connection with the phenomena of plant life, adopting the principle that as two plants from different places, accustomed to different quantities of heat, behave differently when they both receive the same quantity of heat, so also two plants from places having different distributions of rain will behave differently and arrive at the same phase at different times when they are brought into the same place or under the same local climatic influences as to moisture and temperature.

In order to decide as to the limiting value Linsser studies the ratios for the hottest months of the year, which all relate to the ripening phases of vegetation, and finds that for the units of measure adopted by him the value of ratio f/w, that represents approximately a dividing line between the stations that have an abundance of rain. in summer relative to the summer heat and those that have little rain relative to the heat, is 1.2. I have indicated in the preceding table by the letters A and B the stations that have f/w>1.2 and f/w<1.2, and which Linsser puts into his zones of abundant and scanty summer rains, respectively.

I give in the following table some of the more striking and permanently important results of Linsser's computations. His original work, based on about 30,000 observations, gives for each of his 31 stations and for 118 species of plants and for each of the three phases— leafing, blossoming, and ripening the ordinary phenological constant or sum total of mean daily temperatures above 0° C., and also his own physiological constant, which is the ratio of this sum total to the annual sum total for the station. In the following summary I give the physiological constant as it results from the average of all the individual stations in the zone A; but for the sake of quicker comparison between the results for zones A and B the summary gives not the physiological constant for B, but its departure or difference

2667-05 M-15